^^ Yep. Cadwell is basically offering you the chance to experience the storylines of the other alliances with the same character in a sort of "alternate timeline" situation. You can return to your original zones at any time via Wayshrine.

RobbieRocket wrote: »

So, it's a great thing for completing achievements without actually having to make a new identity...?
I presume there are no negatives regarding this, compared to making an entirely new character in another alliance.

Yup, pretty much. The big difference between taking Cadwell up on his offer and starting a new character in another alliance (well, aside from the fact that in one you'd be starting a brand new character, and in the other you're keeping your character, which is actually a massive freaking difference) is that when you do the zones for the next alliance as part of Cadwell's Silver, the zones are scaled to VR1-5 (so the starter islands and the first full zone are VR1, the second full zone is VR2, etc) rather than being scaled to the levels they would be if it was your home alliance. Once you complete that alliance and move on to the third alliance (Cadwell's Gold) the zones are scaled to VR6-10.

It's also worth noting that until you complete the quests required by Cadwell's Silver you can't go freely around Tamriel, because the zones of the 3rd alliance are still barred to you. You also don't get to choose the order in which you do the alliances after yours - instead it moves counter-clockwise around Tamriel (so it's DC->AD->EP).

Edit: also, if you choose not to take Cadwell up on his offer, you're going to have a really hard time finding enough places to earn XP in order to eventually level up to VR14. You could do it all in Craglorn or Cyrodiil, but it will likely take a lot longer.