Re-studding a load-bearing wall
I'm doing a complete remodel of an 8 x 8 bathroom. As part of the demo, I took out an extra interior wall that limited the available space for the vanity to 36". It was only after the demo was nearly complete that I realized the purpose of the wall was to disguise the fact that part of one wall is framed with 2 x 4's and the rest is done with 2 x 6's to allow room for the plumbing vent.
After considering my options, I decided that my skill-level precludes moving the vent so I'm going to sacrifice the 2" of floor space and bring the 4" section out to 6" to match the rest of the wall.
I considered just sistering off-set 2 x 4ís but there's also an old frame for a small set-in medicine cabinet in the same section that I need to remove (and possibly replace with a larger one). So, by the time I got everything done, the framing would be a mess.
What I'd like to do is just re-stud that section with 2 x 6's and be done with it. The bathroom is located on the top floor of a split level home with an unfinished attic above. The upper part of the wall in question is an exterior wall with double top plates and studs 16" oc. The span from the perpendicular wall to the first 2 x 6 is just under 42".
My questions are:
 Can I just remove and replace one stud at a time without using temporary jacks?
 Can I add 2 x 2ís to widen the top and bottom plates?
 Should it be done before or after the stud replacement?
 Since this wall supports the attic floor joists, should I do anything to allow for shrinkage in the new lumber?
Any help will be gratefully appreciated!