There may be a prompt, but there's never a mandate to update. New unlicensed versions can be test driven at any time.
For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest.
If you didn't retain your purchased installer, releases that fall within your six-month period can be downloaded from here. Any of them should accept your code.