There isn’t currently a way to move the old posts to the new community, other than just manually reposting them.

The mods of the old community can lock it and make a pinned post directing people to the new one (this caused some controversy when it was done with the android community on lemmy.world). I don’t think that’s necessary in this case. As long as people know about the various Bitcoin communities, one of them will win out as the main one.