I've been doing that here as well. But now that I actually want to use it I'm finding I can't

I have a Blue booking for FLL-BOS and it even says it has a change fee waiver (and a cancel waiver)

I tried to change it to a BOS-DUB flight and it says there are no flights available. If I search for that same date there is indeed a flight and there are seats available in all booking classes.

Is it not possible to change a domestic flight to an international flight? Is this just a web site limitation or is it a rule I've fallen afoul of?