Hello Jo,
Glad to see that your French project is still live.
You shouldn't worry too much about your licence exchange until a month or two before actually moving, as the detailed rules at the Préfectures seem to change from time to time. When we changed my wife's UK licence, they contacted the UK licence people directly but it took forever. A few months later, when we did mine, the rules had changed & I had to contact them, pay for a certificate of entitlement, and get that officially translated into French. OTOH, my licence was issued in 2 months whereas my wife's took 7 months!
For the Dordogne dept. I found (at this site) that there is an email address specifically for driving licence enquiries: pref-permis-conduire@dordogne.gouv.fr
I'd be inclined to try to write in French if you can. A viable alternative is to write in English & use translate.google.com to give a translation that can be cut and pasted into the message, along with the English original. (There are so many Brits/poms in dept. 24 that they must be used to getting that kind of message.)
The practical details of the exchange also vary according to time & place. For my wife, we posted the application but had to go to the Préfecture, with her UK licence, to get the replacement, since our (very) local Sous-préfecture didn't then handle driving licences. A few months later, for mine, I was able to pick it up locally, but they still wanted me in person as I had to hand over my old licence to get the replacement.
Essentially, this is how you cope with the interim period - You physically retain your old licence until the replacement is ready, so you can drive both in France and overseas until you get the replacement. This is why I suggest starting the exchange procedure after being here for about 6 months.
A tip that might be useful, especially for going back to Oz, is that you can apply for a free International Driving Licence (IDP) as soon as you have your French permis de conduire. The IDP is valid for 3 years & issued by the (sous-)Préfecture. As it is a multi-lingual document, it might help with cops, or car-hire staff, who don't speak French, but should always be used in conjunction with your official licence. For driving around Europe, you shouldn't need an IDP, but it might sometimes prove helpful.