i think it happened just before we Empire Squarers moved in in 2006. the story heard the most is that because Tabard Square sounded too similar to Tabard Street and would confuse the postmen! mail going missing, ending up in the wrong places etc etc Plausible...
Tabard Square was the marketing name and can still be seen on some signposts around (which no doubt should have been taken down years ago). The name changed at the time if occupation - seems that the post office as a veto n actual postal addresses. Seem to remember that Godfree Court was also marketed under a different name.
At the risk of being extremely sad, it's not their official address.
If you go on the Royal Mail website (home of official addresses) you'll see that the address is just Unit10-12 Empire Square, London, SE1 4NA.
and vice versa....especially as they are around the corner from each other and Royal Mail in its infinite wisdom deems the street name is not necessary in the offical address relying simply on the the building title and post code....leading to post regulary going astray (when they do deliver of course!) and people turning up at the wrong building (fortunately not far from each other).
Suggest the need to re-instert the street address and the usual RM head in the sand/no problem assertations.