The catch of the 738s being in mods right now: while boarding my LGA-DEN flight last week, the FAs gave the pre-flight announcement about the new wifi (the whole "our new faster wifi for $5" spiel), but the plane (N3741D) hadn't gone through mods yet and it was still $20 for a Gogo flight pass when I went to log in. On the return, no mention of wifi during boarding, yet the plane (N3750D) did have the Viasat wifi.
Guess that doesn't bode well for FAs giving pax a warning when they board those used 739s without wifi that are entering the fleet...