Adobe made a big effort so that migration might be as smooth as you would expect it. but don't be surprised, if there are some issues in your implementation, all depends how you use DTM and what you want in Launch.
However, Launch offers a bunch of new opportunities you never had in DTM (or you would need to use a lot of custom code). but writing all the cases or differences in here would take far too long.
Based on our experience (we have already migrated the first few properties) I would strongly recommend to "verify" your current implementation. You can take the chance to not only migrate, but "rethink" the current implementation. If doing so, the effort might be much more - but worth in the long turn (depending where you start at).
there are a lot of very useful articles I can recommend, just use google to look for:
- https://medium.com/launch-by-adobe/migrating-from-dtm-to-launch-57548251a86d
- https://www.searchdiscovery.com/solutions/partners/adobe/adobe-launch/dtm-launch-assessment/
- DTM-to-Launch Migration Series #1: Options and Considerations - 33 Sticks
- https://webanalyticsfordevelopers.com/tag/launch/
- https://jimalytics.com/tag-management/adobe-dtm-vs-launch/
