You disclose a changed circumstance if a fee held to tolerance increases. Doing so correctly allows you to reset to fee tolerance baselines and avoid tolerance cures. Technically, if you wanted you could just never disclose your changed circumstances as long as you cure the fee increases at closing. The point is, a changed circumstance is only relevant in the context of an increased fee, and only if you want to reset your fee baselines.
A decreased rate does not require a re-disclosure. The only way a decreased rate would be considered a changed circumstance is if the borrower is now paying more points or receiving less credits because these line items are held to tolerance. That said, if the borrower says, "i want a new disclosure to review my new payments" you can disclose this on an LE, or on a CD, whatever you want. If you send the disclosure on a CD, you'd only need to send another Cd prior to closing if some other portion of the regulations require it. Apr becomes inaccurate, another changed of circumstance occurs, product changes, etc.