Of course you can if you made a mistake. It would not look suspicious if you were operating a multi crew aircraft and had previously logged every sector as P2 and changed some to P1/US. Add a note about the reason or better still, with something from your company stating that the corrections changing 588 hrs of P2 to P1/US between 09/06/2011 and 06/07/2012 are correct, or something to that effect.
So at the stroke of a pen and after the passage of months or years you state that the person who thought he was in charge of the aeroplane at the time wasn't, and the person who thought he was P2 was actually in command contrary to what was understood and recorded at the time?
I find it utterly beyond belief that in UK it is somehow deemed acceptable for FOs to log their handling sectors as P1u/s without the Captain even being aware that he isn't actually in command at the time, or that P1u/s only occurs on handling sectors. Heck, why not just write it all as P1u/s? It's a meaningless exercise anyway now the oh so valuable pre-captaincy training tool of real P1u/s was diluted into this farcical and dishonest paperwork fudge.
Given that UK is so dramatically out of line with not only the rest of Europe but also it's previous CAA ANO regs it wouldn't seem surprising to me if we saw some future air accident investigation focussing on morally and logically fraudulently obtained licence(s) due to this fudge.
Surely the CAA is in an impossibly exposed position in requiring 250hrs P1 time for issue of a licence - presumably on the grounds that it is minimum essential experience for licence issue, and then contradicts itself by allowing all but 70 of those 250 P1 hours to be parker pen time. The lawyers would have a field day, wouldn't they?