There is no limit to the number of periods which can be added to the lesson pool. A reduction of the number of periods is only possible down to the number of already scheduled periods.
As long as the lesson pool has not been scheduled, changing classes and teachers is possible without any problems. If a period of the lesson pool has already been scheduled, a change is only possible, if the change does not create a clash. If the change causes a clash, it cannot be carried out.
If the lesson pool periods originated from a substitution or a lesson, they cannot be changed.