I emailed customer service on Saturday and received the following reply:
"Good to hear from you. We're happy to help. We updated your mileage
information as you requested. Your new balance will be reflected online
in 24 to 48 hours. To view your account online, please visit
www.FrontierAirlines.com and sign in at the upper right-hand corner of
our home page."
Sure enough, the missing 1000 posted and now I have the 1500 originally promised.