i plan to go to iad-itm-iad in sept or oct(big window) for 10-12 days. being retired is nice.

i have points to burn, but absolutly no status with anyone. there are many windows of availability in biz class on nh during that time frame. there is no F available. i presume that is held til last minute, as there are only 8 seats in F(and 2 may be for crew). if i book in biz, and waitlist F, and F comes through, does it really cost me 45K miles each way for the upgrade, or does it cost me the differential between biz and F, which is 35K for the round trip. i asked on the ft ana site, and never got an answer.