Here in Australia, P&O and Carnival were almost the same in terms of cruise experience. Of course this changes like any cruise with the particular cruise length.
No doubt P&O will be missed, despite many not liking to sail on her. With Carnival taking over, time will tell if they change things too much but I doubt people who cruised regularly with P&O will just stop cruising altogether. They may not like it as much but I think with no other choice they will just cruise Carnival.
P&O also sourced ships from HAL, I think moving older ships within the Carnival Corp umbrella may still happen, just that they may go to Carnival now. Although they also have sold off ships to other cruiselines as well Azamara, CMV, Ambassador just to name a few.
Interesting, although as Carnival Corp already owns them, it may just be re-badging, and shuffling deck chairs. May mean changes to service and crew as well.