Office 365 Midsize business - need more users!

We renewed our 365 midsize business in December for 12 months (annual payment purchased via a reseller).

Now, 1 month later, we need to add a few more users - but our reseller tells us they can no longer supply midsize business.

I was previously aware that MS were replacing midsize with other plans - but as we use Access databases our option will be E3 which is double our current price.

I'm wondering if this now forces us to move plan, so soon after renewing for 12 months - simply to add a couple of users.