I've got a set of four 12mm spacers. They have the bolts as well as locking bolts. I'm not sure if my locking bolts will be long enough. I'd need to check. I'm assuming you have longer bolts?
I'd potentially look at putting the 15's on the back to push out just a bit more.....
Where are you?
To clarify.. Are you looking at swapping a pair or all four (a set)? Not sure I'd put 15's all round... Just the back retaining the 12's on the front