If we look now the the technologies used in these new generation Airbuses I'm sure airlines would like to replace faster their old A300/A310's. This family and also the early generation B. 757/767 are also in a process of replacing because in all these years lots of things changed. I think for Airbus the fact that they're publishing that you can have a one cockpit convertion from A318/319/320/321/330/340 cannot work out with the latest generation of the A300-600R and the A310-300, so in my opinion A300/310 will soon end the production. About the airline of my country TAP/Air Portugal they're going to replace, starting next year the A310-300 by the A330-200's and I've talked with a TAP pilot that told me that even in the first A320's TAP received ( CS-TNA/B/C/D ) are already big differences compared with the lattest ones they received from last year ( CS-TNE/F/G/H/I/J ) so as you can see aircraft builters are going very fast!!!
I hope this helps!
CV990, the Maserati of the skies!