Stop panicking, you won't get bumped. A guaranteed cabin means you will get 'at least' what you paid for & possibly even higher grade. It is possible you will not be advised until you arrive, that's not unknown, but try ringing P&O direct, they should be able to give you a clue at this stage. Relax...........
I've just phoned again and have a cabin! Phew talk about pre holiday nerves.
(got a small upgrade too!