Can I upgrade my Carrack to an Idris?

No. These are different ships.

I think in the new CCU system, if and only if the Idris is available. Makes a lot more sense.

Yes. The CCU system will cover ships that are currently on sale. So if you can't go in and buy the ship now, you won't be able to CCU to it. So it doesn't grant you special privileges. It just makes existing functionality easier.