I'm in the same boat, so I totally understand the concern here. (I have around 50k flashcards I've created in Anki!)
On the surface, it feels like it'd be a huge loss to stop reviewing all of your Anki flashcards, but as it turns out, most of that stuff stays in your head regardless. For instance, in February or March, I re-started Japanese after an eight month lapse and just suspended ALL of my cards and started over, as if I was an intermediate speaker who just discovered Fluent Forever. As it turns out, it didn't hurt my learning speed in any substantial way (and is a bit more comfortable than trying to review thousands of lapsed cards that I can’t remember). Pedagogically, it’s not a huge deal breaker to simply abandon your old Anki decks when transitioning over to the new app.
So here's my current plan:
- Continue studying with Anki until my target language is available in the new app.
- Around one month before my target language's release, stop making any new cards and reduce my new cards/day count to 1-2. Goal is to get pretty solid with the words/grammar I learned at the start of the month.
- At release, switch over to the new app and treat myself as an intermediate/advanced student. The app will rapidly learn my known vocabulary and customize its level to my needs.