You need to be careful and refer to the footnote on the website (wording is unfortunately not very clear). Note that while DL does not typically hold back available C+ seats from the C+ comp upgrade bucket (SU), they definitely do with FC seats (OU bucket). Just because there are seats are still open in FC and you are within your upgrade window, does not necessarily mean there are any available for comp upgrades and that you will just be able to select one.