Since I didn't see a Dev comment yesterday regarding what is happening to people that are not a cap, I will post again that your proposal of keeping them at their current XP and then having an "unexpected" amount to get to the next level is wrong.
Consider your average, non forum reading player who logs out "1 pip" from level 26 the night before the next expansion hits. Maybe they have a piece of equipment they are going to use, maybe its a new quest they will do at the next level. The point is, they log out expecting that the next day they will run a couple quests and get their level.
Instead, the update hits. They log in. They are now SOOOO much xp to the next level. In fact, they do a couple quests and notice their XP bar doesn't even move. This is going to result in support tickets. And frustrated casual players. These are significant downsides than incur real costs to Turbine (support isn't cheap).
Instead, you should recalculate the xp, so that if I'm 90% done level 25, when I log in after the expansion, I'm 90% done level 25. There are no downsides to this over what you've proposed.