Edition upgrade from x86 to x64 is not supported by Microsoft, nor is running x32 beside x64 on the same machine. The only way you could pull off AC on x64 SQL Server would be to install SQL Server Standard x64 on a new VM then move the AC database to that instance of SQL Server via detach/reattach. You would have to properly name the instance of SQL Server and do your level best to incorporate all the settings into SQL Server manually that AC sets up automatically (security, etc.) and then get AC to recognize that instance of SQL Server by manually editing the XML file and possibly a few more things (which I don't remember off the top of my head). You would also need to burn some incense, pray to your favorite superior deity(s), and possibly sacrifice some small animals in hopes that AC doesn't go as far as to check SQL Server details such as x32 or x64 when it starts up (some apps do) and then refuse to run if it doesn't like what it sees.
While I would not say such an undertaking would be impossible as long as AC doesn't check SQL Server particulars that closely (I'm confident I could pull it off), I would say your chances of being in the good graces of AC tech-support would be == 0 once you are finished. Meaning you and you alone would be solely responsible for all issues with AC from here on out and support would always fall back to your setup as an excuse for why you are having any problem you have and how they can't help you.
So unless you really enjoy living on the wild side or have a skillset equal or exceeding mine, I would recommend following the beaten path of SQL Express x86 to SQL Standard x86.
One day, now that you have my curiosity up, I might play with getting AC to run on x64 SQL Server assuming no one else has ever done it. If I do, I'll publish something.
JamesNT
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