My wife bought an E-series and I decided to try and make its head fit my brush. To my surprise, there wasn't much alteration needed and my Frankenstein toothbrush works better than my wife's.
The E-series heads are much lighter than the A-series. If you look at the the bases next to eachother, the A-series base is a little larger than the E-series. I think the A-series base is a little more powerful in order to compensate for the difference in brush mass. My A-series base running an E-series head is quite a bit more powerful than my wifes E-series set.
Booya. Saved money and got a superior toothbrush.
Step 1: Buy an E-series head
If you don't own any tools, I would recommend that you buy a small carving chisel to gouge with. I just used a typical utility knife. The job wasn't as pretty as it could have otherwise been, but it worked.
The reason you should buy the head first is evident in the next step.
Step 2: Inspect the inside of the head
First, they added a little piece of plastic to the inside to prevent the head from being able to fit properly. Second, the changed the threads a little so it wouldn't fit on an A-series.
On the newer E-series heads, the extra piece of plastic is absent. Maybe they decided that the different threads were enough the prevent people from being able to upgrade their A-series. If you buy your head ahead of time, you can save yourself from having to do a little bit of carving.
Step 3: Carve away
If necessary, now carve a notch from the inside of the A-series base. The best way to do this would be to put mascarra, stamp ink or some kind of washable ink on the little plastic nub. Screw the head onto the base. The nub will transfer the ink onto the base in exactly the right spot. Cut a notch a little larger and about 1/4 inch down.
That should be it.