Firstly, who says he's going to? Secondly, there will be many people carrying the torch; as long as they don't carry it at the same time, the timestream wouldn't have any problems. Finally, where did you get the idea that the timestream can't change? the Doctor has said repeatedly that history can change, and we've seen it happen over and over again. It's only if you try to change a fixed point that it 'messes up time'
- ↑ There is a campaign to let David Tennant recreate the scene for real, but (a) that doesn't mean it's going to happen, (b) even if it did, that would be David Tennant, not the Doctor, and in our universe, not the Whoniverse, and (c) even if it were the Doctor, that would be the 10th Doctor, not the 11th. There's no indication that the 11th Doctor will carry the torch.