Welcome to the Community!
Can you tell us how long ago you purchased the Huawei GR 5? If it's purchased less than 15 days ago and that you haven't used it for more than 30 minutes, you would be able to exchange it within the satisfaction guarantee!
Otherwise you'd be looking at an early upgrade and I can send you a PM to take a closer look into this with you.
You can definitely do an upgrade and keep your existing plan. Just keep in mind that Fido have different plan categories and they each offer a different subsidy on a new phone.
If you have a Grandfathered plan at the moment, it depends in which category it falls. The category of your plan will determine the subsidy that you'll be eligible for.
It's possible that certain devices are not available with a subsidy on a specific plan category.
If you are pass the 15 days and 30 minutes satisfaction guarantee as @FidoPierre mention you will have to do an early cancellation.
To find out how much that fee is just login to MY ACCOUNT click on the number you want to upgrade and on the next page look at the bottom you will see "Upgrade Today Fee:" that will be the amount you need to pay off in order to do the upgrade.
Did you take a peek at your "upgrade today" balance to find out the cost?
The balance of your existing phone is separate from the cost of the new one. The Nova plus is $400 on a PLUS 7 plan. This means it will cost you $ 400 + subsidy balance + $15 transaction fee.
Let me know and we'll take it from there