I was going to type this very thing.There is actually no level 50. Once you finish level 49 you turn Veteran rank 1. There are 14 veteran ranks.
These ranks require more experience to level up (bout 10 regular levels).
You don't actually move alliances, but you are allowed to experience the world through you enemy's eyes. In you case you will be sent to vulkhel guard in stonefalls (ebonheart pact). It will be however a scaled version of the map. All the foes and quests are for veteran rank 1 and only people that started in the aldmeri dominion can play in the scaled version of the map.The second map in ebonheart pact (Deshaan) is for veteran rank 2 and the final map (the rift) is for veteran rank 5. The goal there is to do a quest called cadwell's silver. This is basically all the quests involved in the ebonheart pact story. However, you need to gain a lot more xp to be able to level up and keep "on track". After that you are sent to the daggerfall convenant to do cadwells gold. There you are on copies of the DC maps for veteran rank 6-10. These again are only for AD characters.
Characters who started in the ebonheart pact will go (after finishing the main quest and their own maps) to daggerfall convenant (VR1-5 version of the maps) and then to Aldmeri dominion (VR6-10 version of the maps)
Characters who started in the daggerfall convenant will first go to the aldmeri dominion (VR1-5 version of the maps) and after that to the Ebonheart pact (VR6-10 versions of the maps).