Is abdominal lipo worth it - will it make my tummy firmer/looser? Is combining with an implant exchange a good idea?

Liposuction is one of the most surgeon dependent procedures and can be combined with other procedures as well.  Most patients want to have liposuction that produces a dramatic result, but that also leaves the skin smooth as though surgery was never performed.   These two issues are diametrically opposed, and only an expert can accomplish both.

and if you were my patient,  would be planning the liposuction first and then a reprep and drape for the implants.  Your skin quality will determine whether your skin will shrink or stay 'loose'.  But contours will be made better with liposuction.  So if you want to do this, ask your surgeon about it.  

Yes, you can definitely combine exchanging your implants for an XL size and do liposuction of your abdomen. One of my specialties is XL breast implants as well as liposuction.