What the letter does
A broker of record letter is a one-page instruction from the insured to the carrier: for this policy, from this date, this agency is my agent of record. The carrier re-codes the policy to the new agency, routes correspondence and renewals to it, and pays it the commission. Some carriers ask for their own form or an ACORD 36 instead of a letter; the content is the same.
What it does not do
It does not cancel or rewrite the policy, it does not change the carrier, and it does not change the price. It also does not move policies you did not name; each policy needs its own letter.
The waiting period
Carriers give the current agent a window to respond, usually measured in business days. If the current agent persuades you to stay and you sign a rescission, the letter is withdrawn. Otherwise the change takes effect at the end of the window.
Doing it from the wallet
In CoverMatch, uploading a policy runs the eligibility check on the spot. When the carrier accepts agent changes and CoverMatch is appointed in your state, you see "Good news — we can take this over": you sign the prefilled letter on your phone, the letter goes to the carrier automatically, the policy sits in your wallet as a transfer in progress with a tracker (Signed → Sent to carrier → Carrier review → Done), and from then on certificates, changes and renewals happen in the wallet. When the carrier does not accept agent changes, the policy stays with your current agent and CoverMatch offers a replacement quote instead.