I think it’s a good idea to continue down the path of rebuilding the stationery just to get a clean build with the current version of the program. Fortunately, Designer makes it easy, because I can easily spot which areas have overrides in Style Designer and which don’t. I have the old and new projects open side by side, so it’s not too agonizing. (Thank your Dev team for making areas with overrides easy to spot in the UI.) Just wanted you to know that I’m doing this.
I have been in technical communications for 20+ years, and have yet to encounter a company with better post-sales tech support.