yep IPSec renaming is a pain in the *censored* :\
You can only easily rename an ipsec if you happen to have that FGT in a FortiManager. FMG can do that for you.
If you don't have a FMG you can only do they way Viknesh wrote or old school which would be deleting all references, delte the ipsec and recreate everything which you don't want to do with that many references of course...
"It is a mistake to think you can solve any major problems just with potatoes." - Douglas Adams