I could not find anything on this so here's a post to say it was easy to install Visual Studio 2008 Standard edition after running the Visual Studio Professional trial version. So far no trouble working with my web application, windows forms, windows service, and windows DLL projects that were upgraded from VS 2005 using the 2008 Pro trial version.
I installed Standard on day 89 of the trial. When I started the IDE it defaulted to the Pro trial, so I went to Control Panel, Programs & Features (Vista), and selected uninstall on the "Microsoft Visual Studio 2008 Professional - ENU" entry.
That launched an installer looking essentially the same as the VS2008 Standard installer, but with a "maintenance mode" label. Next, selected Add/Remove, and confirmed yes to "remove all of Visual Studio 2008". The warning sounds a bit ominous, but it removes just the Pro version and after that my opened up in Standard just fine. There was a single warning about a package not loading which apparently can be resolved by running devenv with the /resetskippkgs option.