1. It's a post or column, beams are horizontal (sorry, it's the
engineer in me)
2. Can you just dig the hole larger? If it's concrete, you may have to
break it up and repour some new concrete around your new post rather
than trying to just fit it in the existing hole.
3. as a LAST RESORT, maybe... just maybe, IF you can jack up that
corner of the carport (use a jack and as much of the old post as is in
good condition), you might be able to wiggle the new post in. 12" of
jacking will put a lot of stress on the rest of the carport, especially
if it's attached to the house and not free-standing so I can't really
recommend this as a good idea without seeing the situation. It can
also be dangerous if the post isn't squarely placed on the jack, things
can fly rather quickly if they slip out under load. I only mention it
as an alternative to digging the hole larger. Depending on your
specific situation, digging the hole larger will probably be the better
of the two ideas.