How to migrate all your contacts between two providers on android?
To do that natively only using your phone, you'd need to either manually enter all those 50 contacts to the new provider, then drop the old entries.

You might also be able to export all your contacts to SD card (using the contacts/people app: Menu › Export/Import › Export to SDCard), edit the resulting file, and import it back again – but that might be error prone.

An easier approach would be using a third party solution, like Simpler Contacts Backup – which directly lets you move contacts between accounts, according to the app's description.