git change upstream

Without deleting anything, using git up to v1.7.12:
git branch --set-upstream branch_name your_new_remote/branch_name

Using git v1.8.0 or later:

git branch branch_name --set-upstream-to your_new_remote/branch_name

Or you can use the -u switch:
git branch branch_name -u your_new_remote/branch_name