I was wondering. Right now I am looking at IAH
and using miles. The dates I want are currently available with Standard miles (35,000 RT in Coach). However I am willing to spend the 75,000 standard for Biz class. However I don't want to lose the 35,000 offering. If I book at 35,000 and the standard 75,000 comes up can I spend the miles difference or am I locked in at economy and that is it?