It would be just like the Jin merge. Most people would just get mad and quit because all the time and money they put into their gear is now a complete waste as all their gear becomes junk status overnight, they get offered worthless compensation, and they get forced into the ****ty server they made an effort to avoid in the first place.
What they ****ed up on is they should have put all the PH players into Ran and merged Jin/Isen back in the day and kept Jins xp/drop/forge rates and all the extra stuff Isen offers while keeping it an upgrade free server, but they screwed the pooch on that one already. It kind of sucks now Korea is finally getting around to a class balance and the main server is butt****ed with upgrades lol. If they really wanted to they could make some tweaks to Isen that would make it a ton better, mainly forge/xp rates, but its YNK...they'll just do the opposite of what players want lol.