This is a great question. Let me preface my response by saying . . . please consult with a board-certified physician, who is well-trained in using lasers and can evaluate the particular scar to which you are referring, before deciding upon whether or not this treatment is appropriate for you.
In my experience, fractionated CO2 laser resurfacing (ie. Fraxel Re:pair or Active/Deep FX) actually can improve the appearance of a keloid or hypertrophic scar (if performed on appropriate settings by a trained physician). I have used it on patients with hypertrophic and keloidal scars that developed after a localized trauma to the skin as well as on burn patients who've had a mixture of hypertrophic, keloidal, and atrophic scars encompassing large portions of their body. The use of topical agents (for example a mid-to-high potency petrolatum-based steroid versus 5-fluorouracil) applied immediately after the laser procedure, as well as twice per day over several days following the procedure appears to be key to preventing overgrowth of scar tissue from recurring.
There have been great strides in the treatment of scars using lasers over the past few years. These treatments can be lifechanging to patients who've had large areas of their body burned. Not only is there an "aesthetic" improvement to the areas of their bodies covered with scars, but, functional improvements can be seen as well. For example, scar contractures over joints - which limit one's range of motion - can actually be softened after one or more treatments with a fractionated ablative laser.