I have never done this on a Tonner doll. But on a Barbie you would have to soften the head in very warm water to pull the head off. Good luck hope it goes well :)
I actually boil the water, then turn off the heat and soften the head. Particularly with an older doll. It wont remove the paint, just watch the hair style.
Electric hot pad found at any pharmacy. I detail the process here, http://www.flickr.com/photos/smh223837/sets/72157634849085195/ but all you need is a very old towel, an electric hot pad and time. Because Tonner plastic is more dense, You might want to use a towel on the head when pulling it off (I'm guessing it's going to get very hot). But this is the best way I know to pull off a doll head without disturbing the hairstyle.
I use a dish towel. I use a pair of hot dog tongs to get the bottom of the head in the water. For me hair style isn't such a big deal, because I am usually going to reroot or reset, but I see the heating pad is probably a good idea for ones that you aren't going to do anything with the hair on.
That was also really informative for those new dolls with all the fancy stuff to keep there head on, lol. When I was a kid it was so easy and the plastic was so new then, I used to do it all the time for fun. My mom finally told me to stop, apparently I was upsetting my grandmother. She saw a mad scientist in the future, lol.
LOL, Kathy, I get the same response from my mother. And thanks for all the info, you guys! I posted this a good while ago, and since then I have been using the heating pad method, it's the quickest and the easiest, and you don't have to worry about your very young cousins boiling their doll's heads!