Before Columbia, didn’t shuttle missions swap out crewmembers while they were there? Is it because another shuttle mission was scheduled (but won’t make it now) in september?
Simple answer: The current ISS crew aren’t scheduled to return until October.
They are the 11th crew to man the space station since it opened in October 2000. That first crew was transported on the Russian Soyuz spacecraft. The next four crews went up on the shuttle. After the Columbia accident, the Soyuz performed the next six crew changes.
The plan was to take a new crew member to ISS on the shuttle Atlantis in August. Because the shuttle fleet is currently grounded, that mission may be scrubbed. A Soyuz resupply vehicle is scheduled for late August. Perhaps the new crew member will go then.
The Soyuz craft is much smaller but also less expensive to operate, and has not faced the safety concerns of the shuttle. Typically, the shuttle is used for major resupply missions and delivery of new components which are too big for the Soyuz.
Plenty of details on current, past, and future ISS operations are available from the NASA website.