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.