This is very difficult to answer, as
it depends on the "many changes" you've made. Generally speaking, most sites should be able to follow the
Upgrade Guide or
Migration Guide to upgrade from 7.x to 8.x.
However, if you've made major code changes to either the backend (Java code) or frontend (Angular code), then you may need to upgrade that code to be compatible with DSpace 8.x. In particular, you need to be aware of the "Breaking Changes" in 8.x:
https://wiki.lyrasis.org/display/DSDOC8x/Release+Notes#ReleaseNotes-BreakingChanges You'll need to make sure you keep those "Breaking Changes" in mind if you have a lot of custom code (e.g. custom Java code *must* be updated to Jakarta EE / Java 17, and custom Angular code *must* be updated to Angular 17, which requires "standalone components).
Overall, the
less you customize DSpace, the easier future upgrades will be. So, it's easier to upgrade if your customizations are mostly at the theme level...but much more complex, if you are adding larger amounts of custom Java code or custom Angular code.