First, and most importantly, before looking at your old lists, write down what’s most on your mind right now. If it’s among what you’re worrying most about right now, it needs to be front and center in your new system.
Second, from your old system, move everything to “someday/maybe”, or to a sublist in “someday/maybe”. If it’s on digital, just drag it over there without looking at what’s in there yet. If it’s a paper list, relabel it without reading it yet.
Third, sort the new stuff in from step one onto your now empty lists. Here’s also where you can make changes to your system, what lists you have, how it’s set up etc. It’s even a chance for you to switch system. It should be a rule in GTD to never, ever change systems, but here’s your chance. But if you can get back to your old system, that’s even better. It’s more likely that the reason you fell off it was that your life changed in a disruptive way rather than an issue with your implementation itself.
Fourth, and this is the painful part; finally open your someday/maybe with those old lists and selectively pick the most important old stuff and bring it back into your new lists.