Can i use the windows 8 installed on my HDD with new MOBO

Yes. Windows 8 is quite good when it comes to this kind of thing.
Would still recommend a reinstall, but if you want to carry over your install it should work fine.

What board are you coming from and going to?
 
You would need to reactivate windows with the new mobo because "Windows has detected a hardware change and needs to be re-activated". So what I suggest you do is take down the product key and ID because Windows will ask for it. It happened to me when I upgradeed from W XP to W7 and I didn't activate it so my pc locked, luckily I backedup my data!