From my understanding, the SIM card will become useless once the number is ported out. And it doesn't seem to be a Speakout-only thing, Rogers and Fido do the same.
I'd call Speakout customer service just to be sure though.
To be honest, why not take advantage of the "Buy $100 of airtime, get $74 off a phone" deal that Speakout currently has? You can get your grandmother a brand new phone and $100 of airtime (which she would need to get anyways) as a holiday present.