What a big change for you, your family and your dogs!
I moved from the States to England some years ago so I can appreciate the journey! Good luck with the move and take lots of pics - I've always wanted to go to Australia but I hate flying!
Anyway, I have never re-trained a dog from going outside to inside so can't really comment on how successful that is normally. Hopefully some other members can assist you.
However, I would assume that since your two are already trained, it would be a case of just re-directing your command to their new place. So, for example, if you tell your dogs to "go pee", tell them the same thing at the new place you wish them to go and then reward them for doing it?? That is the way I'd approach the situation.
One thing I know about Yorkies is they are very intelligent and quick to catch on to new training. It wouldn't' surprise me if they picked up on this new training in less than a week and made you wonder what you worried about!