You need to migrate data from a backend with one schema to a backend with another schema. You need to map the data between the schemas, which may include dropping some source data and/or generating some additional data for the destination. Also, if you have (encrypted) credentials stored in the database the other platform needs to support that format. Otherwise you'll be stuck with resetting user passwords. Then there's the issue with downtime during the migration. And probably a couple other things I forgot.
All in all it's not rocket science, but I would strongly recommend getting someone who already has done this kind of thing before and has at least some level of experience with both source and destination platform.