Stage at which EB3 can be moved to Eb2

[H1BANDGC]

I have a query which seems to be basic but wanted to clarify:

My employer is planning to file green card in EB3 category though I qualify for EB2 category but I am thinking that let employer file it in EB3 category and then I can get it moved to EB2 category at later point of time so that I have a PD. Can you let me know after which stage in the green card process can the date be ported and EB3 changed to EB2?

[immihelp]

It is not really like moving from EB3 to EB2. You have to pretty much start all over again, but with the possible advantage of being able to use your old priority date.