The solution in this thread is a bit misleading, since it's implied in the question that the user has a lesser plan. The text quoted below indicates that the app could essentially become invalid, depending on how many features of Plan 2 were used, forcing the user to buy Plan 2 (or the modern equivalent), which is exactly what he wanted to avoid.
Q: What happens when my trial expires?
A: You'll be prompted to request an extension of the trial or purchase a plan 30 days after the trial starts. You can find details about all plans on the pricing page.
If you don't extend the trial or purchase a plan but you still have another kind of license, you can still use all the features of Power Apps that your other license provides. Any data in Common Data Service will remain as it is, and any app or flow that uses Common Data Service will continue to run as it did if your license supports them. If you try to use features that Plan 2 supports but your existing license doesn't (for example, modify a schema or entities in Common Data Service), you'll be prompted to purchase a plan.
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And, BTW, why is this trial for Plan 2 still hanging around in Office 365 to this day when the plan was discontinued last year? It no longer exists, in favor of other plans:
https://powerapps.microsoft.com/en-us/pricing
