Microsoft 365 Business Standard key cannot be applied to existing subscription

Posting this in case it helps others.

We had a Microsoft 365 Business Standard subscription under MOSA that had been renewed for years using product keys purchased from the Microsoft eCompany Store.

When redeeming newly purchased Microsoft 365 Business Standard keys, Microsoft 365 Admin Center displayed:

"The product key you've entered can't be applied to this subscription. Instead, use this product key to add licenses to a new subscription."

After a support case and escalation, Microsoft confirmed this is by design.

Although the product name is still Microsoft 365 Business Standard, the newly purchased keys belong to a different offer variant than the existing subscription and therefore cannot extend it.

Resolution:

1. Redeem the product keys using Create Subscription.
2. A new Microsoft 365 Business Standard subscription is created.
3. Reassign users from the old subscription to the new subscription.
4. Allow the old subscription to expire.

Hopefully this saves someone else the week-long investigation I went through.