Sonicare toothbrushes rock. I was skeptical, but after buying one I'll never go back. I bought an A-series about 4 years ago. My only real complaint was that the head was a little bulky and the inside of the head was hard to keep clean. Then the Elite series (E-series) came out. It promised better maneuverability and increased cleanliness. I was a bit pissed when I found that my toothbrush wasn't compatible with the new heads.
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
Trust me, you'll be able to do this conversion. Your hard earned money won't be blown on buying the 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.