It is a production server - CENTOS 6.1

Some user having root access in the past had logged on to the server and removed the SUID bit from the /bin/su file and then exited root. Now we are not able to switch back to root. Also ssh access for root is disabled on the server, so root directly cannot login to the machine. Since we are nor able to su to root nor able to ssh as root we cant set the suid bit for the /bin/su file back.(Also we are not able to switch between users using su)

How it should had been:

$ ll /bin/su -rwsr-xr-x. 1 root root 30092 Mar 10 2011 /bin/su 

How it is right now:

$ ll /bin/su -rwxr-xr-x. 1 root root 30092 Mar 10 2011 /bin/su 

Is there anyway we can switch back to root or set the SUID bit in anyway ?

Note: We want to avoid a reboot no networking usermode bcas the server is in use 24x7 and getting downtime is a bit difficult. If reboot was possible then we could simply login using single user mode as root and reset that bit.