Forget this idea, it's impossible to do it. Better solution is before Hi5 comes, advertise the servers. I'm 101% sure, that a lot of new ppl will rage both x5 and x15, because it's not a java platform (no hard feelings to l2j, i like it, but it' so different).
So imo advertise is the only true key for both x5 and x15 for "survival". This can be done via the admin/gm team, but better when the players intorduce RPG-Club to their mates.
btw. a lot of admins would kill for the x5 online numbers, even if you compare how many toons were there before x15 opened. 2-3k ppl means a quite good balance both at the market and in playing experience. Saying that a 2k online server is dieing sounds a little bit folly...