[Matt Sowder] “Can I open 640 bins and recapture them as 720×486 or do I have to start from scratch?”
You can redigitize bins and timelines into 720. Select whatever you want to redigitize, set the hardware settings as you want them (e.g. 720*486) and press SHIFT, CTRL, ALT, COMMAND while clicking on batch redigitize selected. Two warning messages will appear that you are going to redigitize in the selected audio and video settings. (CTRL-Shift is for audio and ALT-COMMAND for video I think, but it could also be the other way round). Then you will redigitize into the selected settings.