That would be crazy nuts (and doubtful) if it were a complete fleet swap. Silkair in Singapore (SIA affiliate) swapped out something like 30+ A319/20s for 737NGs and MAXs. Obviously a much smaller number than the Spirit fleet. Would need to be 150+ 737s and a large number of E2s for thinner routes I suspect.

I know - highly unlikely (especially with the current MAX backlog) but no doubt Boeing is working the numbers...