Some airport identifiers begin with a letter while others begin with a
number. What's the difference?

A nearby airport changed from 1F9 to XBP. I would assume that there is
a cost associated with such a change. What is the reason for this
change that outweighs all the costs? As far as I know there is no
change at the airport.

http://www.airnav.com/airport/KXBP

I searched the web but did not find an answer, that made me even more
curious.

Thanks,

Deep