I'm going to weigh in on this one to agree that route-building with the HD grid involves substantially different tradeoffs to route-building with the 5m/10m grids. This isn't as simple as "keep doing what you're doing, just smaller". It will definitely require some amount of experimentation and relearning.
An automated conversion of an existing route is going to run onto this head-on; when you're building a new route, you'll (hopefully) build to the strengths of the implementation, however when initially converting a route you're only going to see the "weaknesses". For example, when developing the conversion process, we initially created a faithful representation of the original terrain heightmap, with the thinking that creators would not want their route to be messed with. It turned out that this is a terrible idea, because the original heightmap was always a compromise based on the limitations of the terrain grid. The current conversion process instead smooths the original terrain, with the end result that the terrain looks nicer than the original in most cases. In some cases you really cared about certain aspects of the original, and these might not be preserved exactly in the conversion. It turns out that this isn't a major concern because the whole point of upgrading is to access more detail, and that means you're going to be editing the route extensively anyway. The fact that there are a few things you'll want to edit after conversion is not a problem, it's entirely the point of doing the conversion.
Texturing is a bit harder for the automated process to deal with. Where you might previously have picked a texture and then splatted it over a relatively wide area, you're now going to find that you can finesse things a lot more by selecting between textures, adjusting tints, using a fine-detailed height brush, and so on. This can lead to some impressive results, but the flexibility obviously comes with a downside - nobody has time to finesse thousands of square kilometers of terrain. To mitigate that, we've added the scrapbook brushes which help you to develop a particular look once and then reuse it as necessary over a larger area. (Important note: scrapbook support for HD grid is very poor in the current alpha build, but this is already being worked on and will improve over time.)
Additionally, where a T:ANE-era route might have relied heavily on ground textures, as of TRS19 we've been pushing the use of Clutter and TurfFX for detailed ground cover rather than relying on texturing. Routes which use these technologies appropriately simply don't require the same number of unique textures, as the textures are merely there to form the base layer and not the entirety of the result.
For somebody building a new route, we don't currently expect that this will be a problematic change. Sure, it's a change. It will take some time to get used to. But we expect that people will adapt quickly and the results will be worthwhile. A big part of the reason we're taking the unusual step of releasing an alpha build is to get early feedback on how people are coping with that transition, and that's why we've talked up using built-in content rather than trying to import existing routes/content.
For somebody who has an entire route built to TRS19/TRS22 standards and wishes to upgrade it to benefit from HD grid, that's a much bigger ask. First, you'll need to learn the new approach. Then you'll need to undertake a trial conversion and see what happens to your route. You may find that you have a good starting point, but you'll probably need to touch every important area on your route with the height and texture brushes anyway, because what's the point of using HD grid if your source material is all at 10m resolution? At a minimum, you'll probably want to smooth off the rough edges near the track - but it's likely that this is only the beginning.
But you may also find that you are regularly going far beyond the new 16-texture limit. If that's the case, the automatic conversion will (at least currently) make quite a mess of your route. Textures will be replaced with something different, often something inappropriate. This is scary at first glance, and it does probably mean that you have more work ahead of you than those who didn't hit the limit very often. However, given that you need to touch every important area on your route with the height and texture brushes anyway, it turns out that this doesn't add as much work to the conversion as you might think at first. The biggest gotcha is perhaps not having the original textures visible as reference while you're busy replacing them. There are a few approaches which might work there, one of which is having two copies of Trainz running side-by-side.
The lack of paint-time texture scaling is annoying but ultimately not a serious problem. As has been demonstrated up-thread, this feature probably caused more problems than it solved. There are some good uses for it, don't get me wrong, but it's certainly not all roses and killing it off doesn't lose much in most cases. What it does mean is that some textures which were scaled inappropriately originally need to be fixed up or replaced. In routes where you're deliberately using an off-scale texture, that possibly means you'll now need custom rescaled textures. We have some options under investigation here, but it's not clear what the timeframe is for those and we don't have any plan to bring back paint-time texture scaling.
The lack of paint-time texture rotation is more interesting. There are three use-cases that I can think of offhand:
1. Holding down the rotate key while painting. This is basically a way of smudging out all the detail in the texture, which is only useful if the texture was bad or inappropriate to start with. It reduces performance and reduces visual quality. We've long recommended against this.
2. Rotating the texture direction from time to time while painting a large area with the texture, to avoid obvious repetition. This is a more meaningful use-case, which is partially negated by having larger textures and better detail maps on the textures. There are definitely overly-repetitive textures out there, and my advice is to just avoid those. Even the best texture can get repetitive if it's used over a large enough area - in that scenario you should start looking to bring in other textures, which you can control much better with HD grid. Paint your own texture or color tint details that don't match the repeats of the original texture. You should also use clutter and turf effect layers to avoid relying so much on the base texture.
3. Following the direction of some natural structures in the terrain. For example, wrapping rocks around a cliff face. This is an area that we haven't fully investigated at this time. We have some great ideas here but they're longer term. We'll be watching with interest on how this goes for you in the short term.
And of course, an important point to keep in mind: the old terrain types aren't going anywhere. Your route won't suddenly stop working if you don't upgrade it. Take your time learning the new capabilities and then determine how it's going to work for you.
Sorry for the long post, but hopefully that's some food for thought.
Merry Christmas and happy holidays,
chris