If I may interject into a family affair,
Whilst the J may have many wonderful abilities, it does not include the ability to procreate. We only ordered 25, enougth to replace half the fleet. Until, therefore, additional airframes are procured, the Ks will have to continue to serve. No one is going to order more new aircraft until the saga of the A-400M is settled; and if the A-400M is selected then the new airframes won't arrive till 2010-12 at the very earliest.
It therefore follows that the Ks are going to have "soldier on" for another 8-10 years regardless of how roles are shuffled around.
The only question to be answered is in which role(s). Which role would you suggest they could take to release
Js to assume the TS task? In the meantime, the upgrade would seem cost effective at £20M in total.