When you port a phone number from one provider to another, your account with the old provider is terminated once your account with the new provider is activated. The old SIM card is permanently deactivated, it can never be re-used.
You can port the number back. From Fido back to Public Mobile. But your old Public Mobile account is already gone, any funds or rewards or add-ons or promos attached to it are already gone, the old PM SIM card attached to it is already permanently deactivated. You'd have to buy a new PM SIM card and activate it with a new (randomly assigned) phone number, then port your phone number from Fido.
Always be sure to request the number port from your new provider (Public Mobile in this instance), not from your old provider (Fido in this instance). The CRTC rules about this are written to favour consumers - you have no obligation to inform your old provider, they'll know when the port request comes through, and they're obligated to refund or pro-rate any balance owing.
Also, if you purchased your phone through your provider then be sure that your phone is not carrier-locked before you leave. This shouldn't be necessary (because it shouldn't be locked), but if it is locked then they'll have to provide the unlock at no cost. If it is locked then you'll have a very hard time getting them to unlock it for you after your account with them is terminated - certain providers (not mentioning any names like Rogers and its subsidiaries <cough> Fido <cough><cough>) are especially notorious for this - especially if you still have any outstanding contract or tab terms, if you haven't fully paid for the phone then you legally don't own it.
Providers know that unlock requests usually mean subscribers are thinking about leaving. And certain providers (again not mentioning the names Rogers and Fido) will often apply rather aggressive customer retention staff to keep their subscribers locked in and locked down. And it can actually be worth hearing them out - sometimes they might bribe you with a better deal that will make you happy to stay with them - just don't lose sight of the deal you're already seeing elsewhere, why should you do worse if they refuse to do better?
If this sort of confrontation bothers you then just remind them that CRTC rules specify the consumer doesn't have to provide any reason at all. Or use the old standby excuse - you plan to travel out of country for a while and want to make sure your phone will work with foreign SIMs.
