This thread contains useful information on same day changes and standby on Air Canada flights. There is a very detailed wiki at the top of the thread that provides a good explanation of how SDC and standby work, both broadly and with many details.
Please read the wiki before posting a question.
Note that this thread is an amalgamation of several previous threads, and that some posts were carved away from older threads. To the extent the thread is slightly disjointed in certain places before
this post (which explains which threads were combined and what was left out), it may be a result of these threads being merged together at a later date. Please forgive any untidiness in these older posts. However, given the comprehensive nature of the wiki, it shouldn't be necessary to read many of these older posts to get a very good understanding of how standby/SDC work.