Yes! You can request a plan upgrade to go from a lower data plan to a higher data plan. For example, going from the By the Gig plan to Unlimited plan. You can also request a plan downgrade anytime during your billing cycle, meaning you’ll go from a higher data plan to a lower data plan. For instance, going from the Unlimited plan to the By the Gig plan.
If you upgrade your plan you can choose to activate the new plan immediately or later. However, downgrades take effect only from the next billing cycle.