major release upgrade in place

[Yasha Karant]

I have been told that SUSE Enterprise Linux (SLES) supports, without 
problems or issues, an upgrade in place to migrate from major release N 
to N+1 without reformatting those parts of the disk / file system that 
contain systems files (e.g., /usr ,  /boot , etc.).  My understanding is 
that this is not possible/safe with EL -- e.g., SL6 to SL7.  Does anyone 
know if this is true -- and works?  If so, why does EL not support the 
same method?

Yasha Karant