My father in law has comcast in Houston, and once he got an HDTV, he just had to take his SD cable box w/remote to one of their offices and switch out for an HD version, and it was free. Same thing for me with TimeWarner, I took my SD box in and swapped out with an HD box. There shouldn't be any fees to do this, just a swap unless comcast in FLorida does things differently.
One note of interest though, this won't get them a cable box with DVR. If they want a DVR then they will have to pay extra each month, probably like $10 a month.