Longer Gamertag for older systems

Hi there, i was unaware of the single change limit on gamertag (that's free). but wouldn't mind it so much except the only game I play uses the old style which will by default append the number.

I have seen that when you want to change your gamertag there is an option for old systems that supports 15 characters at least, I have tested to see if the long gamertag I want is available, it is.

I want to only change that section of my tag.

Background over now for the question.

Can I please change have my gamertag changed for the older systems.

To keep my current new style tag for this I would have to change it to something else and change it back to edit the field I am referring to.

The long one I would want is "TheJammyEmperor"

:)