Assigning vendor bills that are already paid to a project?

I entered a few months worth of vendor bills and payments before remembering that I needed to assign many of them to different projects. I've figured out how to set up the projects in Analysis types, but how do I get back into the paid vendor bills to assign them? This was not an issue in Sage 50, but I can't figure out how to do it in Sage Accounting! Can anyone help?