A grandfathered G5 module however crappy it is will be downgraded to a maxed-out G4 module if you decide to re-roll it under the new system. So there is nothing to lose at all.
That is precisely it

My middle of the road G5 modules (for most of them I just took the first result I got), will be converted to top tier G4 modules in the new system, and I can then start rolling G5's which should quickly surpass my old mods.
If I hadn't done any of this I would have had to start at G1 for every single module (around 100 modules on my 6 most used ships), and I honestly think I would just give up.
I will be spending the next few weeks attempting to G5 absolutely everything I can, locking down my builds as much as possible, and just doing a single G5 roll for my chosen mod. It will save me an INSANE amount of work later.