bfisk,
(And the disclaimer again: it's for practice and system knowledge, not because I'm unable to find the cockpit windows.)
Think you need to spell it out again
Not sure if you can change a fly-by to fly-over in the FMC.
What I've done sometimes to use LNAV instead of HDG SEL is I've created a PBD point along the same axis (i.e. downwind leg in this case) 0.X nm further ahead. This of course requires good "guesstimation" of what the 'X' should be to create a perfect fly-over point.