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.
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 the 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.
Step 2: Inspect the Inside of the Head
When the Sonicare people designed the E-series heads to not be compatible with the A-series, they made two changes to 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
No matter how new the head is, you'll have to remove some threads from the base. You'll need to remove the about 1/3-1/2 turn of threads working from the bottom up. It doesn't have to look pretty. In fact, with a utility knife it's pretty much impossible to look pretty (get a chisel). Just remove a little and try the head. Keep going until the head is able to thread onto the base.
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.
Step 4: Brush Your Teeth You Filthy Animal
Brush your teeth at least twice daily, preferably after every meal. Visit you dentist every 6 months. Don't let the man keep you down.