When you upgrade or install over an existing edition, you need to back up the web.config file. It has specific keys for encrypting passwords, and when you upgrade, new keys are generated which need to be changed back in order to be able to login with former username and password.
Do you have a backup of the old web.config file? If you do, look for something that looks like this: <machineKey validationKey="7AB6323B41ADC8992BDD749C31D68B9BB2C1C1D" decryptionKey="F76D0FEFFGT4F5C12BACDD3E96250E631FE1C2AC405EDRP41" decryption="3DES" validation="SHA1" /> in the old web.config and copy it into the new web.config over top of the existing machinekey statements. There are several of these type of statements you need to copy over to the new web.config, just browse through and you'll see them, copy each one by one and make sure you are replacing the corresponding ones in the new file.
If you don't have a backup of the old web.config file with the machinekey statements, there is a work around for this, but any registered users other than admin and host would also have to be reset in the same way or they would have to re-register. Anyway, this is what you do: from your website, register a new user called "tempadmin" and use the same password you use for your host or admin login. Once you have done that, you need to go into your SQL admin, open the database, find the user table, and copy the contents of the password you just entered for "tempadmin" into your old "host" and "admin" users password fields. Now you can login with the old password and delete the user "tempadmin". It's a pain, but it is a way around it.
Hopefully, they are working on this whole machine key issue so that upgrades will automatically copy the old machine keys into the new web.config file. This happens to a lot of us users, including myself. It's a pain in the arse when it does happen.
Hope this helps,
Rick