I am using IS avid juicy brake mount adapter (185mm) on my present forks but if I buy a new fork which is post mount do I just need to change the adapter for post mount or is the caliper different? Please help clear up my confusion.

You'll probably need something like this

http://www.chainreactioncycles.com/Models.aspx?ModelID=13717

but check with the fork to see what size rotors the post mount is designed for! Most are to fit a 160 mm disc without and adaptor.

Calipers are the same, buy a proper Avid mount and you'll not be faffing with extra washers(there's enough to faff with as stendard)to gain rotor clearance. Approx cost of £7 in most LBS.

Thanks, It would be on a Fox Float.
So it is just the adapter. Hate to have to buy a new brake as well as a fork.