Not sure if this was posted anywhere else, but in case it helps someone, this worked fine (so far):
#1 upgrade Payment form to the latest version in the 1.5 site.
#2 do your joomla migration to the 2.5 or 3 site (or create a new site
#3 Install Payment for in the NEW site
#4 use Phpmyadmin to delete the PF fields in the database
#5 use Phpmyadmin to Export those fields from the OLD database
#6 use Phpmyadmin to import those fields into the NEW database (make sure you change the table prefix in the sql file first if they have changed!)
#7 reinstall Payment form in the new site.
I'm not sure you need to do #3, but I did that and then reinstalled for good measure (#7). All my forms and records came over into the new site. Seems to work fine so far.