Coming from a Loyalty line myself in the past...if you use an upgrade to buy a subsidized phone you will be back on a new contract plus a $40 upgrade fee and will have to choose a new plan and your line access will be $40.

If you want to keep your plan, then you'll need to buy a phone at full price upfront.  To use the Device Payment Plan, you'll have to change plans.

So, to my knowledge, there is no way you can upgrade to a new phone at a subsidized cost and keep the Loyalty line.

You can change to the New Plan whenever you like, but your access charge will be $40 a month if you do, after contract $20, but because you pay no line access charge now, you have to keep that in mind as well.  Also, remember that you own NO DEVICE until paid in full or you complete your contract to the end.