Data migration from Realm Object Server to Atlas Realm Sync

I realise I’m a bit late on the migration path from ROS to Atlas, but I’m hoping to learn from the experience from other users have had doing this.
I’ve read this migration guide at Realm Legacy Migration Guide - Realm Legacy Migration Guide but this doesn’t go into a lot of detail on how the data migration is performed.

I’d like to run through the scenario and assumptions, and hope this makes things clear:

The iOS app is currently in production and uses synced Realms with ROS with the Realm SDK v5.5.0

Q1. If I upgrade to the latest SDK, how do I connect to the existing ROS? My assumption is that this SDK will only talk to the Atlas Realm Sync server. How can this version see the legacy Realm data?
Q2. Is a separate offline process needed for migration of data from ROS to Atlas, or is this done within the client application somehow? I’m thinking that this needs to be done through a server-server process, through GraphQL or some other mechanism.
Q3. If the above is true, the existing apps will only see out-of-date data as they will still connect to ROS.

Thanks for the guidance!

@Nigel_Grange I have the same questions as you. How did you handle Q1, Q2, Q3?