[R] Update R, keep packages

[Will]

Hi,

when updating R I continuously face the problem that I have to reinstall 
packages or copy them into the new R library path.

Is there a smart way of configuring R (.Rprofile) so the packages 
installed under older R versions (different lib paths) are 
included/found by newer R versions (e.g. R site archive).
What problems due to version conflicts/out-of-date packages can arise?

I am working with R on a Linux server.

Thanks in advance,

Will