The Torchbearer EV suit can be upgraded from leveless to MK XII, but cannot be upgraded further. The option to upgrade the item no longer appears in the right click menu, and the Upgrade UI will not allow you to drag and drop the item into the "Item to Upgrade" slot. However, the item can be re-engineered after upgrading from leveless. This happens on all characters, regardless of faction.
A few screenshots showing what's happening.
Here is the EV suit at leveless showing the ability to upgrade.
Here is the MK XII suit no longer showing the ability to upgrade.
Here is the Upgrade UI showing a red box indicating that the item cannot be dropped into the "Item to Upgrade" slot.