On the contrary, people are successfully mixing and matching in practice, and the packet explicitly states that they intend people to do so and will make sure the final version is fully compatible o. the Subclass Level (more 5E > OmeD&D than the other way around, as far as polish is concerned I'm sure) for older books to remain viable. I haven't done a deep dive for all the available options except for the Bard...but all the 5E Bard Subclasses work juat fine with the Bard packet, I can verify 100%. Bardic Inspiration is changed, but the options still work fine.