If you are referring to the MyRewards discount upgrades you received ($1 off at level 1 & $2 off at level 2), no you will not lose them. They will remain in your account for 24 months once you have received them.
In order to activate the upgrades, you do need to apply the upgrades to your plan. To use both upgrade discounts, you will need to have at least 2 slots. We all start off with one slot, the 2nd slot is earned at level 3.
So free free to change your internet plan to suit your needs. To avoid any surprises, make sure to pay attention to the details (including the plan price) before you confim.