In my opinion: not a good idea to convert old photos en-batch.
Recommend: convert (hand-selected photos) one at a time, or just use PV2012 on new photos.
There are more than one reason:
* As you know, process conversion yields very different results (if photo has been substantially edited).
* It takes some time to polish your PV2012 editing skills.
* Often when you are comparing PV2012 results to previous PV2010 results (which you've become familiar with, and like...), you may interpret differences in a more negative light, than if you were using PV2012 without comparing to a previous PV2010 version.
This last one can not be over-stated. Numerous times I have converted and been disappointed, at least initially. Recovered highlights can seem like loss of contrast. Detail in shadows can be distracting after initial conversion, until you step back for a while, then re-look at it...
Also, the process version itself (results I mean) takes some getting used to (even when not comparing to a previous legacy-pv edit). e.g. if you've become attached to the over-saturated shadows in previous process version, the shadows seem under-saturated in PV2012, etc...
Rob