I would have declined if they tried that move.... they tried that with me in the past and I politely decline to change for free. I did recommend SJC and they accommodated without question.
This morning at DEN, seating in F became available again on my original DEN-SFO flight... called a phone agent and they put me back on the flight without issue. That was around 5:30am. At the gate, they announced that they needed 5 volunteers at $800 each. I took it, and they deposited me right back on the SJC flight.... getting ready to board up as it is somewhat delayed. $1300 double dip... great day!!