Downgrade from Premium plan

We find ourselves in the same situation.  I don't think there are any built in methods to convert Planner Premium plans to Basic.  We have about 25 plans to convert with several hundred tasks each so manual re-entry isn't a reasonable option for us. The only solution we see is to export them to Excel, then use Power Automate to loop through task creation actions with the Excel records as inputs.

For what it's worth, I'd like to post a warning here for anyone else considering Planner Premium subscriptions for their organization. The "premium" option in Planner is not premium.  Without automation support (you can't use the Power Automate Planner connector with premium plans), it is a downgrade and should never have been released so half-baked.  Beyond that there are many other basic capabilities missing like support for recurring tasks, task comments, and a task description editor that doesn't look like it came from 1995 (you actually can't cut and paste into it without formatting errors) and the list goes on an on.  Very frustrated our company made the mistake of accepting Microsoft's use of the "premium" label at face value.