Thanks, these are all really helpful comments. The main thing I'll need to think about is (1) if the wheels are due for replacement and (2) whether I would get a significant improvement from a new wheelset or replace with equivalent model and save $$.

For (1) replacement, the answer is yes, they're old and due to be replaced soon - although no noticeable problems yet, I want to avoid being caught out when something important fails.

For (2) improvement, its an interesting comment from merlinextralight that that it may be difficult to gain a noticeable improvement with another shallow section clincher. I hadn't considered that. But if so, I'd be interested to know your thoughts on the differences in feel, performance, etc. between a mid-range and a top-range shallow clincher from same manufacturer with same tyres?

in other words, why buy expensive when i can buy mid-range?

Local bike shops have suggested these options: Ksyrium Elite S; Dura-Ace C24; Ksyrium SLR (more expensive) - and have also suggested that my current wheels are equivalent to today's Ksyrium Equipes. Any thoughts on these options?