You can have Stamps transferred same visit as obtaining extension.
CW you can apply for extension up to 45 days early..
Pretty sure the transfer of stamps etc is done at the extension desks (L desks) so you can make an appointment for extension and they will do it same desk.
Don't forget letter from embassy, if your embassy provides one.
Also notify your bank of change of pp number.
BTW:if you had valid non O-A you could exit and reenter and be given a 12 month stamp.
Thinking you are on extensions.
Regarding the transfer, the visa will not be transferred just information regarding the visa is transferred.