How can I change to a Google account without losing my Trainer Club account progress in Pokemon Go?
I am level 15 and have a Pokemon Trainer Club account but I am sick of the disconnects. I would like to change to a Google account.
Can I link these accounts or transfer data between the two?
You can not switch to another account and keep you data and you can not transfer your data from one account to another. If you started with a trainer account you will have to keep it or start a new character from level 1.
This might be subject to change, if Niantic feels like it, but for now you have no choice.