For the most part, you should just be able to import your rulesets. The jump from 5.5 to 7.2 is not insignificant. You'll need to make changes to things like non-autogenerated controls to be within the guardrails and remove calls to deprecated rules and whatnot. It's going to essentially be the same type of work you needed to do after upgrading your application.
If you were doing that work in the same rulesets you were upgrading, that makes things much more complicated. You'll probably want to put the code into a ruleset version that is higher than the "upgraded" version, so that you can have users make use of the system that you have upgraded/tested/gotten production ready while you do the work to make sure your new development has no issues in Pega 7.2.