Some people do it with hair driers. Some very brave people do it with heat guns.
If you use an oven, just be aware you can't be certain it has the exact temperature you set it to, and that it cycles temperatures a lot before it preheats.
Anything that aggressively breaks in boots (e.g., repeated heat molds, repeated physical deformations while warm) is halfway towards breaking them down.
If it's only 3 hours, and the shop in Sydney is good, I'd let them do it, for now. Because being careful means trying to guess how far you can heat them towards pliability, without damaging them. And if they know what they are doing, any advice they give is worth listening to.
But, after the heat mold, maybe you should just try skating in them a couple weeks or so before taking any drastic measures. (I waited 6 years, which was too long. But mine were way too stiff for me.)