Uninformed telephone agents are all too common, however I believe that changes to a reservation record requires a change fee (if applicacable) and fare adjustments.

Stand-by for earlier/later flight is normally permited in fare rules, for example a discounted ticket I recently purchased (Q fare) includes "REROUTE........PSGRS WITH CONFIRMED TKTS MAY STANDBY FOR EARLIER/ LATER SAME DAY FLIGHTS ON WHICH THESE FARES APPLY."

I hope this helps, welcome to FlyerTalk.