As many have noticed, PRN-24 has recently been assigned to various SVNs during maintenance periods (reference NANUs 2012003, 2012018, 2012024). SVN-24 was decommissioned on 04 November, 2011 (reference NANU 2011088) and PRN-24 was available for reassignment. PRN assignment to any particular satellite is not permanent and may change as needed to optimize the NAVSTAR constellation.
Current plans for PRN-24 include assignment to the new GPS Block IIF SVN-65 for compatibility tests prior to launch and assignment after launch, however these plans may change if need arises.