For me it feels like it’d be easy to do for an international server. Keep all server forums and use them to discuss the game, announce world info in various languages. Send a link to every account to verify a transfer over to the international server. Gold, achievements, bonuses, etc transfer. Run a massive promo for the first conquest and revolt worlds where you could get 2.5x gold and all new accounts get a quad. Let the fun ensue.
Edit: As for whether a US team would compete with an EN team. It depends on the team. As pointed out, US is largely a collective of friends moving from server to serve but some of the US leaders are/can be good. Thing is that to compete with the top tier of EN teams, it'd involve saying no to friends. There's also the fact that if somebody was going to switch, the window is closing fast if it hasn't closed yet. EN worlds are getting smaller and teams are more established. One thing I'll give credit to Nori for is that she recognized when it was time to switch over. Imo at this point US would be best served establishing a team and relocating.