Man, this thread is right up my alley! I'll post some examples, some more may come to mind, but they'll all have corresponding now photos as it's just easier for me to post them that way.
Ok, let's start off with the Carnegie Centre, now a community centre for the downtown eastside but stated out as a library and for many decades was the Vancouver Library's main branch:
under construction:
Carnegie Centre Construction - 1902/2010 by
entheos_fog, on Flickr
1906:
Hastings and Main - 1906/2008 by
entheos_fog, on Flickr
The library moved out in the late '50s when a new building was built and this sat neglected until it was restored and made into a community centre in the early '80s.
1971 and vacant, fate unknown at this point:
Hastings and Main - 1971/2010 by
entheos_fog, on Flickr
It just received another facelift recently.
2010:
Carnegie Community Centre by
entheos_fog, on Flickr