Orthodontists usually put clear bonding on some of the teeth to move them into the correct position when using invisalign. Without those attachment points the trays may not fit the same way.
Having said that, the trays are basically clear retainers and should move the teeth to the position they are designed for. If some of the teeth have been moved up or down during the first treatment, then the trays would try to do the same thing in reverse. You would end up having to get the same treatment all over again, including having the orthodontist put the attachment points on again to pull the teeth back into position.
If the treatment did not involve major movement to the teeth, I guess you could do it again as many times as you want. The risk is that you could damage the roots and jaw bones by doing it.
