Originally Posted by eternaltransit

Depends how strict the controller is - upgrading to J flags your ticket with AWUP, which tells the controller it's already been upgraded with points, so they generally don't then let you do it again. But, controllers have the last word, so they still might let it through.

It also appears on-board, but then, some pursers allow it so it's entirely up to them.

In short: YMMV