Backpack that 'converts' into a briefcase?

[Xrayted]

Look up leather messenger bags. Many of them double as briefcases and are setup to carry a laptop as well. Most have room to store your superwoman garb for your quick change too. Good luck.

Edit: just re-read your post and you said "no messenger bags". Well, then I would still look at a good quality leather (back pack) that will compliment your business attire. No massive amounts of pockets on the outside, rather understated overall. Just big enough to handle your briefcase and laptop, with a little extra for clothes. Definitely match it to your existing briefcase style and color.