Upgrade between premium versions

Hi!

I am about to purchase your plugin for a client – however, they have not yet decided whether they will require functionality to take deposits or not – this might not happen for a couple of months. Without this, the Small Business Plan is sufficient. Do you have options to upgrade to a higher level premium version at a later stage ay a reduced cost, ie. pay the difference? Or would my client then have to pay the full price for the Medium Business Plan again?

Thx