I'm in the middle of a documentary, and I have a large number of 1440 x 1080 8 bit files from Neo HDV. If I upgrade to Neo HD, would there be any upside to converting those original avi files to 1920 x 1080 10 bit files? And if I do, I'm assuming that the Vegas veg would just reference the new files and pull them into Vegas as if they were the original files.

Some definitely prefer to work with square pixel gained with 1920x1080 post, while you don't gain more detail from up-res source, you do gain more headroom and an easier effects workflow. A secondary benefit to running a 1920x1080 timeline means you post elements, graphic, fx, titles all have 33% more resolution what will show in a filmout or blu-ray master. You should make the jump to 1920x1080 for your next project. I'm not sure how Vegas handles if the source changes size mid project -- do some test first.