Apologies - error in second paragraph. Should say:
What would happen to the responses (assuming there are more than 50) when I go from a Basic Plan back to the Free Plan? Also, what would happen to any other forms I had?
Yes you can upgrade/downgrade your plan at any time. When you downgrade from Basic to Free you will only be able to access the first 50 responses (anything more will not be accessible unless you upgrade again). Any forms you created will remain (with the 50 response limit) and you won't be able to create more forms.
When you downgrade from Basic to Free no data is lost. So when you come back later and upgrade from free to Basic you'll see everything you had before downgrading.