The tab credit would only build up if it already is building up with your own phone as no changes to your account are required for switching devices. all that's involved with switching devices is putting the sim from the old phone into the new one. If a new sim size is required, then visit a koodo store and they will swap it over for you.3
You will need to purchase a new nano sim card which costs 5$. You will then be able to transfer your account over to the new sim card in self serve, insert the card in your new phone and you will be all set! No need to change anything else.
No, you will not get tab credit for having your own device.