Is there a math formula that I can use to convert those old 5 digit ID's to the 7 digit ID's ?
Typically they just swap radios. They hand over the old one with the old unit id and receive a new one with the new id. Then the old radio gets reflashed and handed out to somebody else. That way they don't have all their subscribers out at once.
I think you gotta start from scratch. I gave up on that in Broward since there was too much moving around. E-9 would show up a few days later as R-20 so I gave up trying to track them.