Closing in Garage door while raising existing header on load bearing wall
The new man-door opening will have to be framed as usual - jack studs / header / cripple studs so that part of the wall doesn't sag. 
 (Of course the header for the door will not need to be nearly as big as the  original for the overhead door, it's span is so much shorter.)  That's simple enough.  

The trick is figuring out a way to temporarily support the portion of the wall where you're cutting out the old header and framing the new opening.  There are a number of methods of temporary support, depending how much room you have to work, but you couldn't really suggest what's best without a clear picture of the existing wall framing.