Foglight never runs two versions of the same cart. Once Foglight is using the new version of the cartridge, it's OK to delete the old version of the cartridge. The old version is not deleted automatically in order to give the user the ability to rollback, if required.
Step 1: Upgrade to the new version of the cartridge. Once the cartridge update is successful, push out the update to all of your agents to use the new version of the cartridge.
Step 2: Double check: go to Admin Agent Status dashboard and see if any of your agents use the old version of the cart. Check the status of the old cartridge (it should be disabled)
Step 3: If old versions are used, try upgrading to the newest version of the cart (then goto step 2)
Step 4: Delete the old cart version
Federation: If you run a federated environment, upgrade the federation master first, and then upgrade each of the children.