Hi @Eric ,
Microsoft is planning to provide a migration path:
We'll make a migration path available along with detailed documentation.
Source: Migrate a gateway SKU
At the moment the only option is to delete the existing VPN Gateway and deploy a new VPN Gateway.
Source: Change to the new gateway SKUs
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten