James,
You need to do one thing first in TS12 and that is to remove any Speed Trees and replace them with either billboard trees, such as Clam's CL series, or Speed Trees that can be updated such as RMM's St-series. The reason is, just like for T:ANE, IDV software didn't allow a direct update between the older version to the latest version, and anything made with the old version can't upgrade.
Once that's done, your route can come in. You will need to adjust lighting, and replace some objects of course. And to answer your question regard grass clumps, and whatnot, this stuff can be bulk-updated using the built-in Bulk-update-delete tool.
My method is to bring the route in separately first after updating the trees.
Check for updates because there have been content updates and improved built-in assets.
Then anything leftover that didn't come in, either download from the DLS due to updated assets, and then hunt in TS12 for the balance.
The process is a bit circular and does take some time, but it is doable.
Here's a route that came in from TS12 to T:ANE then over to TRS19.
