What was the level of (development) effort switching to subscription?

Not too bad technically – Apple handles most of it with StoreKit. Maybe a week of work for the initial implementation: setting up products in App Store Connect, handling purchase flow, receipt validation, restoring purchases.

The harder part was deciding on the model. Freemium limits? Trial length? What to lock? I changed this multiple times. The code changes for each iteration were small, but the decisions took longer.