Win 98 Upgrade

Quick question re upgrading Win 95 to 98. What is the main difference between the 98 Upgrade CD vs the 98 OEM CD? I know you can work around the ‘existing operating system’ with witht he OEM CD by renaming the WIN.COM file, but when doing this, Win98 defaults to install in the dir C:\WINDOWS.000. This would tell me that although I am installing 98, it is not going to upgrade 95 or carry over the existing registry settings. If I install 98 in the existing C:\WINDOWS dir, I assume it will overwrite the existing regsitry. Any thoughts on this or explanation as to why one would use the OEM CD instead of the upgrade CD?