On-Premise Licenses don't expire, but if you drop your Enhancement Plan you are only entitled to the version that was available while you were on the plan.
If you upgraded your environment to a version that you are not entitled to, you will need to re-enroll in your Enhancement Plan. You will need to pay for the missed period (Apr 24 - Apr 25), then pay for 1 year forward, Apr 25 - Apr 26. The lapsed renewal will incur a penalty fee as well.
Re-enrolling in the Enhancement Plan will be much less expensive than purchasing a new license. Whoever your Partner of Record is should have already gone over this with you, if they are aware of your situation.
I encourage you to continue this conversation with your Partner of Record.
Best Regards,
Ben Baxter
Accent Software Inc