The Project navigational menu (I call it a NavMenu internally) is currently set up to be static. Whenever you visit a project's front page, you are "locked in" to that project wherever else you go on the site. To lock another project's NavMenu on to the left sidebar in place of the current one, you simply visit its project front page.
Some people would prefer that menu to react dynamically to whatever content is currently being viewed. Thus, if you go to Project A's forum directly from a shortcut or link or the What's New section, the NavMenu will reflect that project's links and image.