@Matt McCulloch - After reading through the Board History: Versions Help Center article and completing a test of

1. opening a board in one of my Free Plan teams (a board that I am the owner of)
2. making a change
3. closing and re-opening the board (doing this as this is usually the trigger to make a version of the board available for restore
4. moved the board to a paid (Team Plan) team
5. opened the board and saw there was only one version available for restore, which was the version as of when I first opened the board while it resides in a paid plan.

After completing this test, I believe that Miro does not use capture versions of boards that reside in a Free Plan team and therefore you will likely not be able to restore previous versions of a board while it was associated with a Free Plan team. 

However, there may be some scenario where this may work for you. For a definitive answer, you would need to ask the Miro support team who could reach out to the appropriate feature team:

@Matt McCulloch - I should also clarify something.

The post title reads:

Can I restore deleted content if convert to paid account?

While the body of the post reads:

would I be able to restore a board to a previous state 

Board History Versions

As of about a week ago, full board versions can be restored. This feature is called Board History: Versions. As per my previous reply, these versions only appear to be captured for boards that reside in paid/Education plans.

Restoring Board Content

This feature allows for restoring individual board objects and is available for all plans. From my testing, this does work for Free Plan team boards and even follows the board as I move it between plans.More on this in the Restoring board content Help Center article.