Welcome to the community!
Unfortunately, I don't think that would be possible -- if you wish to keep those discounts. Most of the Canadian cellular providers offer subsidised plans. That is, the plans incorporate the phone subsidy into the small, medium, etc plans. The total cost of a phone would be the upfront costs plus the subsidy over 24 months. You can pay more upfront and less monthly, or less upfront and more monthly. The lower upfront cost of a device with an XXL plan is due to the fact that those plans incorporate more subsidy towards a phone than a small plan.
You may be able to switch to a small plan afterward, however, you would likely have to pay a downgrade fee in order to obtain the change. In order to change your plan, you can go to your My Account --> Usage & Services --> Mobile --> Change Plan. If you select a plan containing less subsidy while you have an agreement with more subsidy, it will give you a little warning with regards to the amount you need to pay for the downgrade fee (see here). The downgrade fee would likely be the difference in upfront costs for devices under the different plans.
Hope this helps