Only if the installer actually supports installing from extras as well, otherwise an upgrade may be impossible, or leave the system in a broken state.
This is one of my biggest concern with Extras.
I have no problem with many of the packages I regularly use being moved to Extras, so long as I can use tools given to me to continue managing a mix of packages from both Core and Extras - and that includes upgrades. I seriously do not consider reinstalling (then adding my Extras packages) a viable option every 6-8 months (Fedora's release cycle).
I do not have a problem with an upgrade saying: "You have non-Core packages installed. Please specify repository locations [1] or continue at risk of broken packages."
[1] Either "add" a CD ala debian, or point at a local directory, smb or nfs mount point, ftp server, etc of an Extras repository for the version of Fedora you are upgrading to (and any other repositories come to that).
At which point I really fail to see the point of extras: either it's part of the distro, or it isn't. Labeling it as extras just because it's in a separate CD is useful, because then people can choose which CDs to download and burn upfront, but it serves no other purpose.
I see Core as "packages managed mainly by RedHat employees and essential to a base [desktop|server] system" (latter is horribly subjective I know), and Extras as "packages mainly packaged by non-RedHat employees", with the extra requirement that no package in Core should depend on a package in Extras. Both are parts of the overall distribution, but Core simply has stricter QC and inclusion requirements.
HTH
-- Stuart Children
