Ran into a weird one. So, I've got a big unit of Reanimates with Lingering Dead. A bunch of them are killed; let's say 25 of them for this example. This leaves an entire row (and then some) of empty trays. Because of Lingering Dead, these trays are not removed.
Next, Ankaur Maro comes along and wants to add 2 trays to the back. The back, of course, is behind a bunch of empty trays. But if that happens, when the trays are removed, the unit will be broken apart.
We ended up ruling that two empty trays are added to the back, and two trays worth of reanimate figures are added similarly to the rules for regeneration. I think this most closely resembles the way similar situations (like figure upgrades being destroyed) are handled, and therefore is closest to the spirit of the game. There's no rule telling you to do this at all, though. Should probably be in the next FAQ...