When you finance a new phone you have a number of different options available to you - generally these are around the type of finance (monthly finance installments or lease) or the term of the agreement (12 months, 18 months, 24 months or more)
Usually there are two types of plans in this category:
Standard plans - where you finance the phone, have that phone for the whole term of the agreement and then keep the phone or return it to the carrier if its a lease at the end
Early Upgrade plans - where you finance the phone for a period of time. The difference with early upgrade plans is that they will allow you to upgrade (swap for new) your phone during the term. Some will be very specific - you can upgrade once during the agreement and only after say 12 months - others will be more flexible and allow you to upgrade multiple times during the term.
Remember that some carriers will extend the term of the plan each time you upgrade, so you will be tied in to a new full term. Always check before you sign up
Here's a selection of available Early Upgrade wireless plans