Have you tried this lately?
This is an area I have a problem with in Civil 3D. I have never had styles rename when inserting them into an existing drawing. They, in fact, get ignored if the drawing already has a style with the same name. It only works for bringing in new styles, which sounds like it worked for the OP because his are named different. BTW it also will not bring over command settings no matter what method is used.
If you insert the old drawing into your new template you keep your styles, but ALL civil 3D data gets renamed with a (1).
"Existing Ground" becomes "Existing Ground (1)"
Also beware when inserting existing drawings with pipe networks that have common parts lists. We have had some strange behavior with this also. If you have figured it out let me know I have been working with autodesk support and not getting any good workarounds.
Bruce
Bruce Klug, P.E.
AutoCAD Expert Elite Alumni
AutoCAD Civil 3D Certified Professional
Civil 3D 2023.2.1
Win 10 Enterprise, 64-bit