When should you choose upper/lower?

It is useful when you want repeated upper- and lower-body exposure with a simple four-day structure.

When should you choose PPL?

PPL can organize higher-frequency training neatly by movement/muscle emphasis, particularly when six sessions are realistic.

Can you switch splits?

Yes. A split just organises the work. Changing it can be sensible when schedule, recovery, goals or a plateau make another distribution more useful.