My Canon 40D has a -3 range but my prescription has changed to a -3.75 from -2.75.  I find various dioptic adjustment lens on the Canon web site but no information on whether they fit over the existing viewer or whether you mechanically must replace the existing viewer.  Does anyone have any experience with changing these and should I get a -1 to add on to my viewer or a ne -4 and replace the existing one. Thanks!

I have changed the diopter lens of a 40d and you need to purchase both a new rubber frame and the diopter lens . The new diopter lens will slide up into the groove of the new rubber frame. The existing frame is one piece and the inner frame is not removable to allow a diopter lens attachment. It will not hurt to go for additional  diopter value as the viewfinder's adjuster can compensate. Just add the value you buy to the camera's max value. If the camera will max at -3.0 and you need -3.75 you buy a -1.0 value or greater.

Keith is right as you may be better off with a little wiggle room for the future. This is what I would suggest.
Canon "Rubber Frame E" and "Canon E -2.0 diopter". They should be available through Adorama or B&H  Photo.

Thanks guys for the great response. I somehow overlooked your response when you originally did it. I've been putting up with this a while now and its only gotten worse so I returned to post again and saw your excellent responses. Please forgive my belated thanks!