OK to Reuse Invisalign Trays Because of Drift After Initial Treatment Over?

Aligners are made from a material that degrades over time and can distort due to the forces placed during tooth movement.  You MIGHT find that your efforts work, but they might not.  Worth a shot.
There is no real long term data to prove if this is harmful.  The forces used are pretty minor, but I wouldn't be too worried.

I would recommend going back to your orthodontist with all your old invisalign trays and talking with them about your situation in order to discuss all of your options. Since you already went through the entire Invisalign process, there is a good chance that you will not be charged the same fee all over again. (Most likely, there will be a smaller fee).
Your teeth have shifted and they probably have not shifted the same way they were before, or exactly to the fit of the old invisalign trays. It is possible the aligners may fit, but it is also possible they are not fitting correctly and that will be for your orthodontist to decide when he/she sees you.
It is imperative to move through the aligner process under the supervision of an invisalign certified provider doctor.