You can and should migrate the FMC step-by-step up to 6.1. You cannot move to 6.2 until all managed devices are on 6.1.
However, if you are going to remove and then re-add the ASA FirePOWER module you could:
1. Remove the ASA as a managed device.
2. Take your FMC all the way to 6.2 (current patch level is 6.2.0.2).
3. Re-image the module on the ASA directly to 6.2.0 (NOTE - you will lose FirePOWER services during this time as the module will reload and all deployed policies will be deleted.
4. Re-add the module into FMC and set it as a target for your policies and deploy them.
5. Patch the module to the latest release.