Just to add a little bit of info....
The diopter in the 45 degree prism (at least for the 6000 series cameras but I think its the same except a different base for the Hy6) comes standard labeled as a +1 however its actual optical strength is something more like 8 diopters. I had read this on the photo.net forum and confirmed it by taking my prism to an optician. To summarize - if you need +1.5 diopters correction on the other cameras, then you might need a +2.5 diopter (or a +9.5 optical strength from optician) on the Hy6 Prism. I don't know if the WLF magnifier is the same between the Hy6 and 6008 so can only speak for 6000 series which came with -0.5 diopter magnifying inserts in the WLF. Anyone know if the WLF inserts are the same between Hy6 and 6000 series? If so I may have a few different values.
The Hy6 manual says the following:
"To allow focusing without eyeglasses, the magnifier
of the folding hood can be changed for
one making allowance for the wearer´s visual
defect within a range of +2.5 to -4.5 dpt (as
referred to the dioptric value of the wearer).
The 45° Prism-finder gives an upright, rightside-
up view-finder image magnified 2.5x and
can be used in four different positions spaced
90° apart so that it makes for comfortable viewing
even in awkward shooting positions. As the
viewfinder is mounted, its display is automatically
adjusted for laterally correct presentation.
The 90° High-Eyepoint View-finder gives an
upright, right-side-up view-finder image magnified
3.4x. Its eyepiece can be easily adjusted
from +4 to -4 diopters. Its display is automatically
switched to laterally correct presentation.
The High-Eyepoint View-finder can likewise be
used in four different positions spaced 90° apart."
So, in short I do not believe you could exchange the diopter for the 45° prism finder, but it is possible for the other two finders.