If a previously placed object is updated, how do I force every instance of it to update in an existing map?
It's not a new version. Every once in a while I will tweak one of my existing objects because I get a better idea after I've used it for a while.
Many of my objects are made by an iterative process. Make it close to the idea, use it for a while and then make small changes as experience shows its flaws.
It's done the same as always in Classic Surveyor.
Click on the Tools menu located on the top bar.
Click on Bulk Update/delete utility.
In the left side, click on the get-object, and pick the object you want to replace.
Click that get-object again to unselect it because it'll "stick" and you'll end up putting other things in there when selecting assets from the textures, objects, splines, tracks, and track objects menus.
Select the asset in the appropriate menu on the right, Scenery/spline, or Tracks/track-objects, or even textures if you want.
Drag that object into the Replace with section of the bulk update utility.
Located on the lower left of the interface are settings for entire route, selected area, or single baseboard.
Once selected, click the Begin button.
A progress bar will appear. This process can take a very long time depending upon the number of objects being replaced.
The only objects you can't replace are bridges, MO crossings, fixed-tracks, and industries.
Surveyor 2.0 is more complex as usual and I'm not at my PC to verify the steps to perform the procedure.