Changing remote URL doesn't update submodules

If you create a project that has a relative submodule path. Say this is hosted in your personal github space:

https://github.com/username/mainproject
https://github.com/username/subproject

The subproject it part of the mainproject with an entry link this in the .gitmodules:

[submodule "subproject"]
path = subproject
url = ../subproject.git

Using relative URLs has many benefits, a primary one being the checkout works over any protocol.

If you setup a jenkins job to build mainproject everything works fine, if the project is then move to a different github account (say to your employers organisation):

https://github.com/organisation/mainproject
https://github.com/organisation/subproject

Changing the URL the jenkins jobs works for pulling in changes from mainproject, but when submodule updates are attempted the old url still gets used. This seems to be because the origin remote in the mainproject repository doesn't get updated with the new URL and so all the relative URLs remain relative to that project.