Thanks for the update. When GBG was first released it was reported that you needed to do that first battle before moving or else you would end up fighting in your new era. Either it changed or was misreported, but in any case was simple to do and much better to be safe than sorry barring any further reports. I certainly didn't want to chance it when I did my big 11 era jump.I saw this a few days ago, and decided to test it out, since it’s the first time I’ve heard about it. Just aged up to TE today after GbG started, without doing anything battles in GbG. As I suspected, this is incorrect. I’m still fighting CE troops, while the rewards are all TE.
TL,DR: no need to to do anything, as long as you age up AFTER GbG start, you will fight previous age army, while earning current age rewards.