I went to purchase an on-line reservation today, and before displaying the available itinerary selections, this form appeared:
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">
Upgrade Preferences
Select your upgrade preference, then click CONTINUE.
Only show me fare classes that I can upgrade immediately (Y, B, M, H, Q, K)
Show me all fare classes, including classes that I can upgrade within 24 hours of flight departure (all fare classes, including L, U, T)
</font>
Somehow I remember a 72 hour rule for H, Q, & K fares - but this might not be until next year?
GMF
p.s. Strange to be scheduling a second DL flight this month after so long of an absence.