When I replace castors on an upright I always use cast sockets rather
than the pressed ones.  It is important not to change the height of the
castors, so replacing the wood in that area could mean that the new
castors will not end up being level with one another.  Some castors
have cardboard shims to level them.  If you find shims, they should be
put back in place before installing the new sockets.

If the wood has been chewed up and the screws are loose, I drill out
the old screw holes with a 1/2" Forstner bit.  I use Titebond to glue
Dilignit (pinblock) plugs into the holes.  I make the plugs with a plug
cutter (from Woodworker's supply).  Any part of the plug that sticks up
is leveled off with a chisel.  The centers of the new plugs are drilled
to accept the screws.  This method does not change the original
configuration of the wood.

After the new castors have been installed, turn them about and see if
they rub on the bottom board.  It is often necessary to cut away some
wood so the castor can revolve 360 degrees.

Paul Manganaro