The band fits perfectly, but the sides stick out like jodhpurs used to in the old days. That fashion was loose at the hips and tight from the knee down.  Of course I am not talking about the below the knee thing here, but just the hip area flaring out.  If there is enough room to get the sewing machine foot in below the band and take in the hip area with ease, then the band does not have to be unpicked, HOWEVER if it was me doing the alteration and I wanted to take in from the point where the side seam is joined to the band, then I would unpick the band just enough that I can start sewing from the very top.  It does give a better finish to the alteration.

If you don't want to unpick the band, then begin sewing just below the band.  If the sides are over locked together the seams should be laid flat towards the back.  that means on the right side I would sew from the top down, and the left side from the bottom up to the band.

Happy sewing

Judith aka genie