If a warlock with the Book of Ancient Secrets invocation swaps it out and takes the Ritual Caster feat, can they cast already-copied rituals?

Question

I have a player that is playing a warlock. He wants to go Pact of the Tome and take the Book of Ancient Secrets eldritch invocation at level 3, but then replace it at a later with a different invocation. He keeps the book and at a later level takes the Ritual Caster feat.

Would the Ritual Caster feat permit him the ability to ritually cast the already transcribed rituals even if they are not of the chosen class from the feat?

Answer 1

No

First, the book from Ritual Caster is not the same book as the one from Book of Ancient Secrets, so the spells would need to be copied from the latter to the former before the could be used.

Second, Ritual Caster is limited to a single class' spells so only spells of that class could be copied.

Book of Ancient Secrets is considerably more powerful than Ritual Caster.