Well, Lucas has been talking about doing them in 3-D publicly since 2005, which was well before the current trend really kicked in hard. I know that a lot of people see this as him jumping on the bandwagon, but I really don't. They've always said that the future releases would be dependent on each one doing well, so this is really nothing new or anything to worry about. If TPM does well, then we'll see AOTC. If it doesn't, then it doesn't. I'm not sure what their measure of success is, but I assume they need to at least regain the conversion expenses (and presumably make more money to be used for future conversions and the TV show). And John Knoll has stated that the originals will be easier to convert to 3-D since they are more simple visually than the prequels; it doesn't seem to be an issue of going through the old digital files and just re-shooting the film as you seem to be thinking.