You can auto upgrade 32 bit to 64 bit
You can assign a 32-bit and 64-bit package to the same group when you do an auto-upgrade and both clients will automatically upgrade. You don't have to create separate groups for 32-bit and 64-bit clients.
Upgrade clients to SEP 12.1 by Auto upgrade feature
http://www.symantec.com/connect/articles/upgrade-clients-sep-121-auto-upgrade-feature
Note : You must test the AutoUpgrade process before you attempt to upgrade a large number of clients in your production network. If you do not have a test network, you can create a test group. You can add a few non-critical clients to the test group and upgrade them by using AutoUpgrade. You can confirm the upgrade completed successfully by verifying the version number of the client software that appears in the About dialog box.
It's always recommended to have Symantec Endpoint Protection Manager and SEP clients on the same version.
Public Kb:
Upgrading clients by using AutoUpgrade
http://www.symantec.com/docs/TECH96789