The WikiTree ID, such as Green-49771 is the backbone of the entire WikiTree system. These will not change so much - but they can be 'modified'.
For example, when you a merge a profile, this creates a redirect to the new profile. For example Houlihan-1642 is merged into Houlihan-180. If you try to go to https://www.wikitree.com/wiki/Houlihan-1642 it will automatically redirect you to Houlihan-180. As long as the redirect chain is not too long, you are free to link to WikiTree profiles without worry.