Wow there is alot of errors here.
Okay, 17" RS were made early days in Germany with the primary production of all RS, these look exactly the same as 16's, 15's etc. but scaled up to 17's (ie. no large lip unlike the ones in Wabits first pic)
Later on down the track bbs produced larger barrels and lips to fit these older 17" RS, making them 18" (pictured in Wabit's first pic, they 17 centers with 18" OEM barrels/lips.).
All RS, 14/15/16/17 have different centers, however, you can buy aftermarket OEM lips/barrels to essentially upsize (by 1") some of them from the smaller centers.
I am not sure what sizes these are available for, but think you can make the following combos:
15">16"
16">17"
17">18" (confirmed)
Now, as for (actual) 18's and 19's, these are called Super RS and have been built later in Japan by BBS.
These are a totally new wheel only 2 pieces, center is mated to barrel.
Super RS are still being made by BBS Japan and very very expensive. (thus why it's taking me so long to get mine... :( )
Sizes are quite limited and are all 5x114 PCD, a set of 18's will set you back around 5k US and a set of 19's around 8k US.
Now, as to get a bit back ontrack, to the poster, I think you are steering a little off track...
Making 15's to 16's (99% sure you can do it) but, it will be very expensive and your better off buying another set of actual 16's.
By the time you buy 4 barrels, 4 lips and four sets of bolts you will be up for more cash then getting a pair of fully genuine 16's and even if you go the upgrade yours will no longer be genuine... (apart from centers).
I would say either stick with what you got as 15's or sell them (to me :) and buy some 16's.
Also, you have 15x7 at the moment, which means trying for 8.5 you need 3/4" longer barrels and lips, I think lips will be doable as they make 1/4" increments, but barrels would be damn hard to come by...
Anyway, I hope I haven't missed anything...
Aaron.