I'm sure some will be able to swap for free but I've been offered 70% PX value of one mono-riser for the other which seems reasonable to me (I lucked out with a very good deal on the bike price though so I'm less fussed by a few extra costs!). I got the impression this was the standard Cervelo arrangement from my LBS....

I'm just waiting on confirmation of price and availability which I should hear in the next day or so. I'll pass on when I do.