Perfectly reasonable to ask you to pay for upgrades. When you buy you get upgrades for 12 months without paying any more. After that you can continue to use the version you have got forever.

As said above, I guess you tried to upgrade without a subscription. If you don't have what is needed to go back to a previous version I expect that the decent people at Naviter will be willing to help you if you email them.

They need support to continue development of their software. I suggest paying the sub.