There's no mandate to update, and new features can always be checked out by downloading the latest, unlicensed. Just retain the old license, which should be valid if you want to reinstall the version that you purchased. It should fully perform as before, and not expire. There's no need to pay again.
If there are issues, use this form... https://www.nch.com.au/support/reg.html
Export may be disabled after the trial period non-registered versions. Try the AVI format for export. Or install the free, limited feature release linked above, and try AVI.