I don't know how old you are, Segilla; however, these types of sayings are much older than twenty years. I'm using some literary liberty here when I say that Shakespeare once suggested Richard III as crying out "My kingdom is short of one horse."
I think there is a subtle difference, though I may be wrong. "It's up to you" suggests that you may make a choice, whereas, "It's down to you" means you must make a choice, alternatively, that the outcome is your responsibility.
'He/she is as much use as a chocolate fire guard.' My grandma used this expression in the 1970's and she was in her eighties then. An older friend used to say 'he/she is as good as a chocolate teapot'. Don't expect there are many fire guards around now but maybe a few more teapots.
So what about 'he's got all his chairs at home' meaning, I think, he's crafty.
Encounterd this in Accrington, but they speak their own lingo there. Love to know if anyone else has come across it.
BlackadderV.
While accepting that there can be a difference between 'up' and 'down', (After he chopped the tree down he chopped it up for firewood), if I'm right about the more recent appearance of 'down to you', it has become universal. 'Up to you' is rarely heard (except in my home!).
If the subtle difference had been felt to be a necessity would the change have had to wait till towards the end of the 20C before making its appearance.
(A bit late, but...) Mosaic: funny you should mention that one (which I'd never heard) --- in German they say "He hasn't got all his cups in the cupboard" meaning "not all there"!