Can my child go back to a regular school / college at any time during open learning?

Surely. So many children have moved freely between school/college and open learning. Getting an admission is not so difficult in today’s world where schools/colleges far outnumber the demand.
Most schools do not insist on TC (Transfer Certificate) when they know the child has been doing pen learning. They may however demand an entrance test / interview – for which if required the child might need to prepare in advance.