I guess I should see which way uses least code space too, but I just tryed to complile the first suggestion and it did that OK. I'll only know if the code runs as expected later today, when I burn a PIC and try it.
I added this to my code.
Code:
    lAC2        var Long            'Long variables for calibration values actually negative in my sensor
    lAC3        var Long 
    lMB         var Long
    lMC         var Long

'Convert unsigned PBP Word var to signed PBP Long var.

        lAC2 = AC2                           'copy word to long   
        if AC2.15 then lAC2.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true 
        lAC3 = AC3                           'copy word to long
        if AC3.15 then lAC3.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true
        lMB = MB                           'copy word to long
        if MB.15 then lMB.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true
        lMC = MC                           'copy word to long
        if MC.15 then lMC.HIGHWORD = $FFFF 'Check for negative, set top 16bits to all 1's if true