On the Affiliate page, in the "Referred agencies" table, the current commission percentage is listed per referral (column shows 30% or 40%). If you've just crossed the threshold of 10 active referrals, you'll see all active referrals automatically being updated to 40% — no action needed.
The upgrade is permanent as long as you keep 10+ active referrals. If the number drops below 10 due to cancellations, existing 40% referrals remain at 40% (you don't have to worry about falling back).