If you have not upgraded the database it should not be any problems.
The best way I think should be to accually remove the EPiServer nuget packages (by GUI) and do not remove anything that is not EPiServer.
After that you install the packages but from the console, like this:
Install-Package EPiServer.CMS.UI -Version 7.19.1
This should work, but If you have updated the database, you have to manually look in a old version of you database and do a schema compare, or contact support
Unfortunately I have upgraded the database as well. Is version 8.0.0 stable enough to do full featured website on it.
I agree with Quan, it is absolutly stable since it not prerelease any more.
It is often not a problem doing the upgrade, most sites does not need any extra work at all.
So, keep have it as version 8 and put the time you thought spending on downgrade on testning the site instead.
Is it possible to downgrade Episerver CMS 8.0.0 to 7.5 without breaking the site. By mistake I update all the nuget packages to 8.0.0. Will the downgrade cover all the dependencies.