I believe those were Force. As far as converting a 9.9 to a 15, you are probably looking at least at carb/exhaust changes. I would start out with a repait manual and parts list for the two models and go from there. I'll poke around and see if I can find them online. I imagine you might be able to scrounge up the necessary parts on eBay or from a salvage yard, and the labor should be fairly easy.
*edit*
Looks like you can get diagrams/parts listings on Mercury's site at this link, using your serial numbers( example 0E129612 for a '95 15hp):
http://www.mercurypartsexpress.com/Part ... e.asp?CID=
You can also get info at Sears using your model number (225581507 for the 15hp model) at:
http://www3.sears.com/
Enter the model number and then click continue after its strange search results and you will hit the parts/diagram page for the motor(s). Looks like you can get a new carb from Mercury for just over $100, and the same thing from Sears for about $144. New intake manifold for around $35, possibly new reeds and such, etc. It adds up at new prices. After the carb/intake I would expect exhaust parts would need to be fitted, but I can't find a listing from them. Basically you are looking at bunch of parts for intake, a few for exhaust. My target would be to strip these from a broken motor, as new parts would just not be cost effective. As a side note, you might be able to squeeze a little more juice out of your 9.9 by simply changing the jets in the carb, but I would carefully monitor the temp of the engine and not go too crazy with the jet size.
-Dave