The most accurate way to determine head is to measure it.
What is your filter PSI? Multiply it by 2.31 to get return head.

Suction head you can get with a vacuum gage (Sears has them for $20). Multiply in mg by 1.13 to get suction head, then add together. Once you have total head you must scale it by GPM of the new pump. Unfortunately, it is an iterative process because GPM and head are dependent upon each other. I use a spreadsheet with hydraulics equations to solve for each.

For comparison, I have a 21,000 Gallon pool with 2.5" & 2" plumbing, filter, heater, spa and 1HP Northstar pump. From measurements that I took, my head is about 56' @ 92 GPM.

Also, if I had a bit more information, I could estimate head for you and what it would be with a different pump.

Current Filter PSI
Equipment list
# Suction lines, ~ length & dia
# Return lines, ~ length & dia

The rest I can guess.