In what year does that happen?
From what you said there I'm thinking this is a possible scripted event, not something we need to change on the map. IF "Germany dead" THEN give Memel -> Lithuania.
I used such events in a mod i was preparing. Specifically it was for Shattered World scenario where, for example if South and West Germany would get destroyed then East germany would became Germany, would get a german flag, would get all research from South and West Germany and would merge loyalties of all 3 regions into one. This was all done through conditional events (which are great addition for modding btw.). The conditions applied to other regions as well, for example any Italian region would become Italy (with all the things mentioned before) as soon as they are the only region left in that country.
Event seems to work fine but there was a bug i ran into with loyality changes. I'll recheck it in this version of the game and make a separate thread if that bug still exists.