I would imagine the 68s are on borrowed time at this point (if all options are taken, there's enough R211s to replace ALL remaining B Div SMEEs) so at this point in there service life it isn't worth the expense to upgrade the R68s to do that. Also they could just retire the R68s before the R46s, 46s have digital signs and more modern couplers.
Most of the general public doesn't realize the R46 is actually older than the R68s, so if they scrap the 68s first, won't have to worry about passengers complaining, etc.