There is always been a lot of confusion about what is being compared on a BRG claim. The actual rate and rate plan on the original spg booking is irrelevant.
What will be compared is the lowest available rate on spg.com against the lowest OTA rate (comparison can be made with different amenities, but always apples for apples). If the claim is approved, your booking will be adjusted to a new rate plan — the BRG rate plan — and the new rate will be 75% of the competing matched rate.
It is irrelevant whether your original booking was prepaid (rate plans BAR1, Z*H, Z*R, etc.). Once approved, the rate plan on the reservation will be adjusted to the BRG rate plan which almost always (some resorts excepted) is a fully cancellable and no-deposit-required rate plan. Of course, if the deposit on the original reservation was already charged, I seriously doubt the property will refund it, at least not while you have an upcoming booking with them. (There is some anecdotal evidence on FT that, for some resorts, the BRG rate plan is noncancellable with the cancellation penalty being assessed on the basis of the RACK rate for the dates in question).
Now, I understand the LNF allows other possibilities for claims that SPG’s BRG doesn’t, which in theory seems to be very appealing. However, I tried twice and failed to get approved both times.