You can increase the number of periods of an LPL as you wish. A reduction is possible only until you reach the number of already scheduled periods.
You can change teachers and classes of LPL's easily as long as they are not scheduled. If one period of an LPL is already scheduled then a change is only possible if it does not lead to a clash. If a change would result in a clash, it is displayed in the upper part of the window and the change is rejected.

If the LPL's originate from a substitution, they cannot be altered, of course.