Quick thought regarding the Discovery and any other similar situation.
Seems to me that if they want you to sign away your royalties retroactively or they’ll remove your music, the answer should be a resounding “no,” and here’s why.
Setting aside the precedent and principal, looking at it practically if you say “yes” you’re giving them what they want for free. If you say “no” they now have a choice – either follow-up on their threat and remove your music, which costs them quite a bit of money and the end result for you is the same as if you acquiesce, no more royalties. Or they don’t follow up on their threat and you continue to collect your royalties.
Either way there’s nothing to gain from acquiescence.