The deletion is only if you didn't migrate. The migration took place to transfer your game data from EnMasse to GameForge. You don't keep your game data on your local computer. It's kept (currently) on GameForge's servers.
Alright, thank you, i had to be sure since i had issues trying to transfer the account, like many others commenting on here, for example opening the gameforge website after accepting the migration, it wont let me log-in/register with my EME email account, i tried many times, until it let me, and i had to see up in the browser bar saying "migration/completed" to know that it worked finally, because once logged in gameforge website it doesnt show at any place that you successfully trasnferred an account, and it could be taken as you just created a new gameforge account instead xD

