YMMV, but my experience in the past year, purchasing main cabin: As solo traveler, I have been upgraded to First 80% of the time in the D-5 to D-1 period, and a couple of times at the gate (yes, I know... a high upgrade percentage, but as 3MM I am pretty high on the priority list and I try to avoid ATL). With a companion I have been upgraded to First about 40% of the time in the D-5 to D-1 period, and if there was an upgrade at the gate it was for one seat. No significant difference in time of day, etc. for solo vs. +companion flights. 30+ flights, so the difference is becoming statistically significant.