It's a one time puchase of whatever installers you download during the time that your Pro benefits are current.
ie if you pay for 6 months' worth of Pro then whatever installers you download during those 6 months are yours to use for as long as you like (providing you save the installers). Likewise, if you pay for 6 months' worth of Pro, you'll have free updates for 6 months.

If your Pro benefits aren't current then you can't get free updates. Your Pro's way out of date...4.8 is really old. If you want to update Pro, you'll have to buy it again. Your email address will only be in the database while your Pro benefits are current Yours would be long gone.

You're better off trying out the free basic version, you might find that you don't like all the changes since 4.8. Or you might find that you're happy with the basic version.