If I can raise this header higher off the floor, My questions would
be:
1. Do you build the replacement header first, install it, then cut
away the original,in that order?
2. Cut the original out first, install the higher header next, and
move on. How long do you have before the floor above collapses in on
you if you do this ;-)
3. If I raise the header all the way up to the floor plate above, with
no space, then what is the difference in removing it altogether? Since
that plate is supported on both ends. This span is about 50 inches
long.

The biggest question is which way the joists run inthe ceiling
above, and whether they terminate on top of this wall. You
need to support the ceiling above while you take out the
beam the posts supporting it, and the studs above it, then put
in taller posts, and put the beam back, high enough to be clear
of whatever you're planning. That would be far easier to
accomplish if you were openning the ceiling up, too.
Then you could just prop up beams on either side of the wall.