upgrade path from 2008 hyper V cluster to 2012

Dear friends,

i am not sure if i am on the right forum.. and my appologies if this has been discussed earlier also...

i have windows 2008r2 cluster on two servers .. serving production hyper V clients/ vms ... and when i look at the features provided by 2012 cluster and hyperV i feel like upgrading it ... but i am not very sure whats the right way to do the same without disturbing my current clients .. because i cant afford to have any downtime .. please advise if this is possible and how... Thanks for  the help and time ..:)