At the moment, we are not set up for this exactly.
Here is what would happen:
You would add a new, upgraded plan.
We would stop your current plan and add the new one.
When you upgrade, you keep all of your unused credits. (Not like downgrading when unused credits are lost.) You would then get the new additional credits for your new plan. The billing date would then update to match the new plan.
are on the Dash plan at $25 per month
and want to upgrade to the Locomotion plan at $50 per month.
stop your Dash plan, keeping all your unused credits;
add your new Locomotion plan (no prorating);
and start billing you $50 per month for the Locomotion plan.
Your billing date changes to reflect the date you began the new plan.
For more information about changing your plan, check out this help desk article.