I do not know the cost, but Mac said it was about 1800. I would assume that the upgrade kit includes a new upgraded board, and a double sided reader. I believe that Unitec gets about 1200 for a rebuilt board. Your current dial up modems will work with the upgraded board.
What I would suggest doing, is one of two things.
1: send your board in to be upgraded, if can can handle the down time
2: If you cannot handle the down time, then buy the kit, and send your current board in after the kit is installed to be upgraded. this way, you have a back up board for you to use, or I guess you could try to sell it.