is there any way to migrate my username
Yes. After logging in over yahoo account change username on iso before entering a lobby. The old name is now free. Then log in over google account and change username to your old one.
and the thousands of games I've played to that account?
No. Not on isotropic. (At least not without the help of Doug.) The games played with a Open-ID account stay connected with it forever - whatever names were used. (I have used at least 4)
On Councilroom the games are only connected with the used iso-username.