Hi Joe, if you have been wearing the aligners regularly and just slacked now, you can go back 1-2 sets and try. Normally they should fit fine. Best would be to see your dentist and they could advise if you really need mid course correction.
One of the important parts about Invisalign treatment is to remember that you're not paying for Invisalign from Align Technology but paying for Invisalign treatment with your doctor's office. Depending on the contract you have with the office, mid-course correction may be included or have an additional cost.