Is there a big difference? I recently bought the 2012 Flux DS30's and the only difference between them and the 2013 DS30's are the high back and the ankle strap seems to be an asym style on the 2013. Worth the extra for the new strap, and or highback?
I had a pair of Burton Malas that had a regular strap and then updated to a set with the asym strap. You can definitely tell the difference. Better cushioning and support with lateral movements.
"Is it worth the upgrade?" I'd concur with Mrjimyjon. If you already have your '12's paid for then just hold on. If you like the new colorway AND want to try the asym strap AND the new high back then just see what you can sell your '12's for. If the difference in cash it would take to get your '13s wouldn't be too much then go for it!
If you rode them back to back you might notice the difference. Is it a $100 difference, only you can determine that. The one thing about flux is they're comfortable! I have the 2012 rk30s and have no complaints about the straps.