The big problem with upgrading this script is that moving between versions without testing on a separate sandbox site is like playing Russian Roulette with 5 bullets. Moving between versions can be tricky as noted by Max Potential.
There are lots of settings and if all those settings aren't reflected in the upgraded site then all sorts of things can go astray, the db scheme is far from consistent between some iterations of the software, new settings may not be replicated properly on upgrade from old database versions - you need to thoroughly test everything with a copy of your site elsewhere before applying those steps to a live site.
Last year I gave this advice
https://gfy.com/showpost.php?p=18357916&postcount=14
Phpprobid isn't really the software for an adult flippa if that's what you're trying to create. You'd be better off starting with a stable, custom solution. If you're serious about the project then a custom solution makes far more sense than a third party script with known problems.