Hi I purchased the celestron nexstar se8 not to long ago from you guys but now i am looking to upgrade the scope to the celestron C14-AF XLT tube the question is the dovetail mount the same as if I could use the C14 to fit onto the SE8 mount ?
Forget it. The dovetail would work as long as you order the proper tube and dovetail combination. The BIG "BUT" is weight. Your manual that came with the Nexstar 8 should give you the weight capacity of the of the mount, I don't know that offhand but I think it is about 11 pounds. The reason I bought my Nexstar 6 inch was for portability, I have an 8 *inch HD. The Nexstar 6 including mount and tripod only weights about 31 lbs, your 8 inch will be slightly more.
The optical tube that you are considering weighs 45 pounds by itself. The Nexstar mount would be wrecked if you tried mounting the heavy tube on the light mount,
If you really want a 14 inch scope, order a suitable German Equitorial mount and matching heavy duty tripod combination. It will not be very portable but the components could be handled by one person.