@Deji_Apps Non-sync realms and sync realms are of a different storage format so in order to migrate to using a sync realm you would need to open two realms in the app, the old non-sync realm and the new synced realm, and then copy the data out of the non-sync realm and insert it into the sync realm as new realm objects. From there you can then close and delete the old non-sync realm and just continue using the sync realm going forward. Sync realms behave just like non-sync realms in terms of APIs - querying, writing, and notifications are all the same so hopefully the real meat of your application code will not need to change.
The biggest difference is that you open a sync realm with a SyncConfiguration and need to login a user in order to sync data but most other APIs remain the same.
You can quickly compare the differences by looking at the non-sync realm quickstart -
And the sync realm quickstart -