>>You can keep SQL Server 2008. You need to install SQL Server 2012 separately, don't try any upgrade option, just do a clean installation. Your other SQL Server installations should not interfere.
>
>But I don't need it. That's why I was trying to do it all in one shot but I will have to perform a clean uninstall and then a new install.

I don't recommend upgrade. I suggest clean install. As for don't need - do you have 100% control over all environment you're going to install your software? I would not start from uninstalling.