You can’t convert a Mavic Crossride hub to 12mm as far as I know, so you’d need to replace it. You should be able to re-use your rim and spokes with a new hub, so I’d speak to your LBS and see if they can do this for you.
I use Hope Pro 2 Evo hubs with a 12mm rear, but there are others available. The Pro 2 Evo is £130 from CRC, and a complete wheel is £180 ish, so you have to weigh up the options of just buying a new wheel. (I bought my Hope Hoops from my LBS for less than CRC list them for).
Hopefully someone else can give you more information about different hub brands though..