Can I downgrade as well as upgrade as necessary?

JotForm looks great, and a vast improvement over Wufoo. Once each year we take a great many payments (between May - July) for the British Chess Championships, and we'd need to upgrade to 'Unlimited payments' to do this. However the rest of the year is comparatively quiet, and Silver (100 payments) may well be enough. Is it easy to upgrade AND downgrade as necessary?

That is possible, you can upgrade and downgrade as necessary, you simply need to go to your billing page: https://www.jotform.com/myaccount/billing 

Let us know if you have more questions, we will be glad to assist you.