They (the ancients) used to move stuff around periodically in an attempt to thwart graverobbers. There was one tomb, in the Valley of Kings I think, that had a whole bunch of mummies stored in it for safe keeping.
Actually, no, there was nothing taboo about it. The egyptians themselves removed almost all of the Pharoahs' sarcophagi and moved them all into one tomb after the fact... to prevent their bodies from being destroyed by looters who had already invaded their tombs.
It would be especially not a problem for them to open the tomb of someone associated with the Aten heresy of Ankenaten and his children... and their servants. There are other signs of opening of these tombs to desecrate Aten symbology, destroy the cartouches of Ankhenaten, etc.
Also, it is quite obvious to later researchers that the short reign of Tut in a time of depressed economy lead to his relative poverty as Pharoahs go... and that many of the treasures in his tomb had been TAKEN (borrowed?) from other tombs and had the "serial numbers filed off" so to speak.