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. Invisalign covers us for this in most cases but dentists may charge small fee to cover their time and material costs.
Mid-course correction cost depends on the contract with your doctor
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. The doctor has to provide more treatment than originally planned and also has more work to produce another ClinCheck simulation. Best speak to your office to find out what is involved in your situation. Good luck!