Discovery will only pay out the MSA on 1 of 2 conditions:
1. You cancel your discovery membership.
or
2. You move to a plan without a MSA. The downside of this is, you are only allowed to move back to to a plan with a MSA after 6 or 12 months (not sure but it was at least 6).
So your MSA will probably just be transferred to the new plan, if it is a plan with a MSA.
and like the rest above said, you get taxed on the payout.