Just wondering if there is any harm in adding a Custom2 Top Member to an account after it already has data in the [None] member. I suspect as long as the [None] member is within the Top Member, I should be okay, but I want to be sure.
You are correct.
Just make sure that "[None]" is part of the new Custom2 sub-hierarchy. Additionally, you can add a noinput rule in order to block data for the C2#[None].