I did just this last Saturday at Kasikorn in Central, Khon Kaen.
Do take your passport with you. Onto your new phone down load the app through your phone's service provider, eg DTAC, AIS, not through your home wifi. You'll have to buy a days worth of internet access which cost me 19 baht.
I took both phones into the bank. Effectively what they do is to cancel the old one, then enrol you in the new one. You'll be taken to the ATM where the assistant will set it up and ask you to input the PIN number, twice. I used the same PIN as the old app and it didn't complain.
Overall took maybe ten minutes.