Edit Software Distribution Path in Existing GPO
Modifications and transforms are applied at the time of assignment or publication. Hence, they can only be added when the GPO software package is created. This is a Group Policy limitation.
So, in this case, you should create a new package using the Advanced option, identifying the old package in the Upgrade tab and adding the appropriate mst(s) in the Modifications tab. This should upgrade the existing installations on your network.