I see three issues here:
1. Active members who go to a different server, thereby leaving the guild.
2. Inactive members who have quit the game but earned renown.
3. Guild renown owned by the guild because previous players left the guild for whatever reason. (Gone, but not forgotten)
Guild renown has always been just that: "Guild" renown. And the number you get for your character by hovering over the guild renown bar is "player contribution." So, from my perspective, the logical thing to do would be to take the number that the existing server serves up every time you open the guild tab and look at that "total guild renown" bar, and that number moves with the guild to the new server.
The "player contribution" is essentially a bragging right--look how much I've contributed to the guild. While it's nice to look at it and go, "Oh, without me the guild would be half the size it currently is" (or whatever percentage your player contribution is), the bottom line is, you took all those guild renown end-of-quest/saga rewards to build the guild, and the renown is "owned" by the guild. Otherwise, you would have chosen some other reward.
So, the guild should retain its level on the new server if considered from that perspective. The personal guild number could be repopulated when you join your guild on the new server, but it is relatively unimportant to the guild itself. That would address the "gone, but not forgotten" player contributions and the "inactive and probably never coming back" player contributions to the guild's renown that would otherwise be lost in the move.
That does NOT address when active players go to a new server and join a different guild because of game experience reasons (all the ping stuff). If there is a way for their "player contribution" to help out a new guild (many people may be starting over), then I think that should be explored.
In the meanwhile, I think that when a level X guild on a 32-bit server moves to a 64-bit server, the level for that guild should still be X, NOT X-missing players. Because that punishes the guild for all the historical work it has done as players moved in/out/through by subtracting the full 100% earned by those players rather than the current 10 or 25% loss guilds experience.