If the test goes ok, then you will know the "worst-case" performance for the existing heads.
But for the new heads, with a run of 100 feet, you're going to lose a fair amount of pressure.
From what I've seen, there's not a huge price difference (not for a small project) between 1/2", 3/4" and 1" PVC pipe and fittings. The serious price jump starts when you get into the 1-1/4" and larger pipe fittings because they are used less frequent.
So I would suggest that you use 1" PVC for the 100 foot run as a minimum. 1" pipe will have only about 1/4 as much pressure loss as 3/4" pipe. Why? Do the math. A 1" circle has about has twice the cross sectional area as a 3/4" pipe. That means a given amount of water flows twice as fast in a 3/4" pipe as a 1" pipe. Friction losses in pipe are a function of the water speed squared. So when you double the water speed, you quadruple the losses.
You can check out the results here
http://www.hunterindustries.com/resources/pdfs/technical/domestic/lit091w.pdf. If you look at the page for PVC pipe, you can see that at a flow of 5gpm, a 1/2" pipe will lose over 8psi/100ft, a 3/4" pipe will lose a little more than 2pis/100ft, and a 1" pipe will lose a little more than 1/2psi/100ft.
And even if the existing pipe you are connecting to is only 3/4", you still will get better performance in the new heads by using the 1" pipe. Water is relatively a non-compressable fluid. That means it doesn't mater if the pipe is 3/4", 1", or 2' in diameter. The pressure will be the same everywhere except for pressure losses. Basically, the water moving through the pipe creates friction as the water moves along the pipe surface. This friction expresses itself as a pressure drop down the length of the pipe. So using the example of a 100ft long pipe, if the water isn't moving, the water pressure in the pipe is the same everywhere. But if the water is moving at 5gpm, then the pressure at the end of the pipe is 8psi lower at the end of the 100ft pipe compared to the start of the pipe (assuming the pipe is level, because gravity adds or subtracts about 4psi/10ft of elevation change).