Upgrade MRR is the monthly recurring revenue generated when subscriptions are moved from existing plans to higher plans. It takes into account the increase in the MRR caused by factors such as add-ons added to the subscriptions. It’s indicative of an increase in the subscription quantity or movement to a higher dollar plan.
If a subscription is moved from plan A (MRR $50) to plan B (MRR $150) then the Upgrade MRR would be $100
Increase in upgrade MRR indicates your product is scaling as your
Added Hint: Also seeing churn numbers will tell you if there is a value problem (high churn, low upgrades) or a pricing problem (low churn, low upgrades)