Was actually wondering how this merge would work...
Guessing severs on the same time line, released at the same or close to the same time will be merged so you would not have to worry about the players from the new server(s) having a high level gap.
But wonder if they completely rename the resulting server or just give it one of the merged servers' name. Main issue is "how many" servers will get merged together, maybe it's not just 2.