Hi there,
That will depend, are you talking about merging your old gPotato account (and if so, have you already merged your old gPotato account onto a new my.com account)?
If you have not, then you don't have to change anything, the normal process for merging the accounts can be followed (as self merge process is nolonger supported by webzen/gPotato)
Just fill in the support ticket here:
http://support.my.com/games/kbPage/54/ and the support will merge your old account onto your new my.com account.
(please note, if you have made any new characters on it while waiting for the merge, these will be deleted, as only fresh accounts can be used for the merge process - if you would like to keep any of these new characters, please create a second my.com account and use that to merge the old account on)
If your not trying to merge your old gPotato account, and would just like to change the login email for the current account, then please send me a PM, and I will see what I can do for you.
-Wraith