I think you could easily do this by using find to locate the indices of all values lying in the shifted range. This would be something like:
idx = find(y>= 80 & y<=100)
but with the limits tweaked appropriately to set the exact range to be adjusted.
You can then use these indices to alter the relevant portion of the x data using something along the lines of:
or whatever the appropriate value to shift is.
I'm sure there is much fancier and more automated ways of doing this but as a first approximation this should do a pretty good job.