since you've packed for months on the road you need to either pay for the extra baggage you'll be charged by whatever airline they buy tickets on or leave your stuff behind. Air Atlanta does not pay for extra baggage for it's crewmembers.
My contract allows for up to 20kg excess baggage. Other contracts aren't the same though.
Unless you're hired as a Captain, forget about upgrading.
I was promoted from FO to Captain in 2005. This has slowed of late though, for sure.