CentOS - Apr 2006 - Yum curiosity: --obsoletes with update, useful or not?

Got a wild hair this A.M. and decided to inundate my two brain cells.
"Man yum" includes this:

update If  run  without  any  packages,  update will update every
       currently installed package. ...
       assumed to be a shell glob and any matches are then installed.

       If the --obsoletes flag is present yum will include package
       obsoletes in its calculations - this  makes it better for distro-
       version changes, for example: upgrading from somelinux 8.0 to
       somelinux 9.

Hmmm. Cell number 2 doesn't forget much. I recall several admonitions
that major upgrade should take the form of a new install. E.g. CentOS 3*
to CentOS 4*. The paragraph above *seems* to indicate that a major
upgrade might be doable? Moreover, it prompts the questions:

1) "What are the downsides, if any?"
2) "Has anybody tried it recently?"
3) "Have any results to report?"

No pressing need, just curious.

TIA
-- 
Bill
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William L. Maltby wrote:
> Got a wild hair this A.M. and decided to inundate my two brain cells.
> "Man yum" includes this:
> 
> update If  run  without  any  packages,  update will update every
>        currently installed package. ...
>        assumed to be a shell glob and any matches are then installed.
> 
>        If the --obsoletes flag is present yum will include package
>        obsoletes in its calculations - this  makes it better for distro-
>        version changes, for example: upgrading from somelinux 8.0 to
>        somelinux 9.
> 
> Hmmm. Cell number 2 doesn't forget much. I recall several admonitions
> that major upgrade should take the form of a new install. E.g. CentOS 3*
> to CentOS 4*. The paragraph above *seems* to indicate that a major
> upgrade might be doable? Moreover, it prompts the questions:
> 
> 1) "What are the downsides, if any?"
> 2) "Has anybody tried it recently?"
> 3) "Have any results to report?"
> 
> No pressing need, just curious.
> 
yum will upgrade pkgs, but not all pkgs like that - eg. moving from
kernel2.4 to 2.6 the 'centos way' - moving from devfs to udev, major
python + rpm upgrade in itself.

check list history from early 2005, this move from centos3 -> centos4
was documented and thrashed out a few times.

plus, packages change - not everything from el3 made it to el4 - and
what about non core installed pkgs and install from source pkgs!
-- 
Karanbir Singh : http://www.karan.org/ : 2522219 at icq