An R 4.5 app can be read with newer clients without any issues(*) Layout regions were only an issue if put into a design accessed by an older client.
If I didn't understand the question, please clarify.
It's also pretty easy to test out.
(*) There may be code issues; R4.5 didn't have things like the 'split' function in LS and may apps included a custom function to do string splitting; this can be problematic if run on a newer client. If you have may old apps, I'd recommend buying TeamStudio's Migration Filters; they're perfect for finding code issues across versions. I don't work for TS but do use and love their products and support.