Although I have had more LVC's, than I can count, I have never had a pair of 47's... They have always been a bit boring in my mind.
But I do know, that the 47 used to be undersized by 1", but that has changed, as far as I know. So if you are a true 34" and you want to wash your jeans, then I suggest, that you go up a size in the 34", however if you have the 47 in a 34, then I'm quite sure, you can do the same with the 54.
I don't know, what you are referring to, when you say "downsizing ala..." The more recent 55s are oversized by 1" to 2", the older ones weren't - they were TTS.