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

Mostly due to laziness and failing to wear my retainers, my bottom teeth have shifted. Not as bad as they were when I began treatment, but enough that I know I need to do something. I have started wearing my old aligners, starting with one that almost fits the shape of my teeth now. I am concerned, is this dangerous to move through the aligner process without the help of my orthodontist? I do not have the money to go back and pay again for this process, so I am hopeful that using my aligners will correct this problem.