R help - Jul 2007 - installing, removing, upgrading, and downgrading packages

I'm very new to R, trying to learn it.  I started with R 2.4, but I have
since upgraded to 2.5.0, on WindowsXP.  I understand that 2.5.1 is now
available.

Last night in the course of things I loaded libraries "coin" and
"survival".  I received warning messages that they had been built
under
version 2.5.1.  However, as far as I could tell, they worked OK.  But
this brought some questions to mind:

I found the remove.packages() command.  Is there a way to revert a
package back to a previous version without removing it?  If not, and I
remove one version, how do I specify downloading a particular (earlier)
version?

Was that warning of any consequence?  Are there times when a package
will not work properly with an earlier version of R, and will this be
obvious to me when I try to use it?  Or in a more general sense, are
there foreseeable circumstances in which an older version of a package
would be necessary, rather than the newest one?

Should I keep my packages in a library folder outside of the R install
folder?  Right now, it appears that all the packages I download get
installed into a library subdirectory under my R250 directory.
Advantages and disadvantages of either method?

Thanks.
-- 
Christopher W. Ryan, MD
SUNY Upstate Medical University Clinical Campus at Binghamton
40 Arch Street, Johnson City, NY  13790
cryanatbinghamtondotedu
PGP public keys available at http://home.stny.rr.com/ryancw/

"If you want to build a ship, don't drum up the men to gather wood,
divide the work and give orders. Instead, teach them to yearn for the
vast and endless sea."  [Antoine de St. Exupery]