Thanks. They got back to me straight away. For everyone's edification, if you book a prepaid rate you simply email their billing dept with your conf# and they will remove destination fee and charge you appropriate amount. So it won't involve a credit.

Also - for those traveling to SF, Park Central is offering a deal where you get 3,000 points per night and 20% off rates until May 18 (Fri or Sat night required): Code MRP:
https://www.marriott.com/hotels/hote...ncisco/1343623