Prefix this with: not that this matters at all. There have been a number of recent instances of democrats moving to the Republican Party. From a procedural POV is there anything stopping a democrat from switching to the Green Party, PSL, or the like?
Nothing stopping you, the only slight disadvantage is the inability to “influence” (by voting) the majority party in some blue states in the primaries, which is something Dems used to have before they took their masks off.
Some just stay in the party to vote in the primaries and then vote for other parties in the general elections, but it’s perfectly rational to not even bother with that because Dems are incapable of learning anyway.