You'd be far better off throwing your old 121.5 in the rubbish (and for god's sake disconnect the battery before you do this so people don't end up initiating SAR action ending at the dump)
Do NOT do that.
Hand your old ones in to either the Police (a responsible authority), AMSA may take them, or even a place like Battery World as they will dispose of them properly for you- they even advertised it I believe.
It is the only real way to garuntee that your old beacon is 'safe' when it is out of your hands.