Shifting a base EB to an AV could potentially make them easier; AVs only have Boss-level mez protections, but their Purple Triangles make them nearly mez-immune most of the time. EBs, however, have higher mez protection than Bosses or AVs and typically take three stacks of mez to be affected. Presumably there's no way for the game to give an EB shifted to AV rank purple triangles, just as the game doesn't strip the purple triangles from an AV downgraded to EB. With the lower inherent mez protection of the rank, an upgraded EB would be mez fodder for pretty much any character with decent stuns or holds.