Hi there! Glad to hear you’re considering the Omni Sync Server. You can transition over simply by changing your sync settings on one device to point at an OSS account, then updating all your other devices to use that account. For example, on the Mac:
You should be prompted for your new OSS password; enter it and continue
Once that’s done, repeat the process on each of your other Mac or iOS copies of OmniFocus: enter the Preferences or Settings in that app, choose Omni Sync Server, enter your credentials, and you’re good to go! Since all your copies of OmniFocus are already syncing with each other, they should continue to do so with your OSS account without any trouble.
If you do encounter problems in the process, our support team is standing by to help! Send us an email and we’ll do everything we can to sort out your sync situation. (It also never hurts to check the backups of your database beforehand, just in case! On the Mac, choose File > Show Backups… to see all the recent checkpoints of your data.)