One of the larger projects I am working on recently decided to switch from Assembla to Redmine for project management. There are many advantages to Redmine versus Assembla the largest being cross project management for Redmine and in Assembla projects are completely isolated. There are some others that our team finds extremely useful including the stuff to do plugin for Redmine that allows for drag and drop priorities for tickets. It is also going to be nice to finally be in complete control of backing up our data, making changes, uptime, etc.
The below specifically applies to Assembla however can be used for any trac project that is on a remote server. At first I was under the assumption I would have to install trac, have it up and running, and then import into Redmine. This is not the case as the Redmine task will use the directory structure to do all the importing so you only need to backup the trac directory on the remote server and then copy it to the new Redmine server.