Heres an explanation with diagrams from Focussing
The text is a little "Chinglish" but understandable together with the diagrams.
Focusing Screen--How to adjust focusing screen--
The shims are placed in between the underside of the prism and the focussing screen.
AF function is not affected by the focus screen or shims.
If you focus the subject in the viewfinder to be sharp, but the image is focussed closer to the foreground this is
front focus. To correct this you need to fit a
THINNER shim in place in between the prism and the focus screen.
If you focus the subject in the viewfinder to be sharp, but the image is focussed further to the backround this is
back focus. To correct this you need to fit a
THICKER shim in place in between the prism and the focus screen.
so to answer your question " If you replace a thin shim with a thick one, will this bring the point of accurate focus"...
closer to you (ie you will be correcting a back focus problem)