Which FI are you referring to? Probably BMO, right? You're responding to Coolio's post where he's asking about the TD same-card numbers, even when PS'ing between base and premium cards.
TD will usually NOT change the CC#, and the only time they ever did was when I PS'd my old TD Cash Back MASTERCARD to become a TD First Class Travel VISA card, so naturally. Otherwise, no other TD PS'ing that I've ever done has ever changed the CC#, and this includes going from TD basic Cash Back to Cash Back Infinite, and TD Aeroplan Visa Infinite to a basic TD Rewards Visa.
For BMO, all PS's that I've done have resulted in a new CC#, including my up/down switches between the basic and World Elite Air Miles card.
BTW, did BMO give you an upgrade offer for the basic to World Elite Cash Back cards, for the 10%/FYF promo? Or did you call to get the offer. They wouldn't give me the 10%/FF offer on an email upgrade. All they offered via email offer was FYF. So, toss!!