I'm asking how easy/difficult it would be to upgrade?
How easy is... what? How easy is it to update the PHP binaries from one version to another? In most cases, it's pretty simple - even if you're accustomed to building PHP from source rather than just grabbing it from your distro's normal repo via a package manager.
How easy is it to examine every PHP script in use under the old version and ensure that the PHP upgrade won't break those scripts without making any changes? Depending upon a number of variables (no pun intended), that could certainly be a non-trivial task. The best way to proceed is the common method of deploying the change to a development server, testing out your code base, and only once you've ensured everything works as expected will you then make the change "for real" in your production environment.
It's really hard to say "what will it take to upgrade?" without seeing the code. If it wasn't written using best practices (which, at the time, may have appeared more as "forward-thinking" practices), then you'll probably run into problems. The PHP language has grown and evolved over the past few years (IMHO, at least), and the "best practices" have of course adapted to these changes over time as well.
I want to know if there is anything that's keeping them from upgrading.
Change. That's what stops a lot of people from doing a lot of things. From management's perspective, upgrading PHP can take time (time == money) that will probably yield little to no gain for the customers/end users.