Mets and Yankees both announced their ticket credit/refund plans today.
2020 Regular Season Ticket Policies
Mets 2020 Season Updates
Lot in there but as it pertains to plan holders, you can either opt for a full refund or a credit which is applicable to future games or next years renewal. If you take the credit, you get an additional 10% credit which can be used towards purchase of additional games or as a discount for your renewal for the Yankees. For the Mets, you get a 20% credit which is valid towards additional games, an upgrade for your 2021 plan, parking, or concessions. So far, the only games being impacted as part of this policy is March and April.