I have a Windows 2008 R2 x64 print server and have successfully added and shared about 30 printers. I've also successfully added the x86 print drivers for all these printers to allow our users still on XP to use them.
However a situation has come up where I need to update the x86 driver for one of the printers. When I go to the printer properties and select additional drivers the x86 option is already checked and grayed out.
My conclusion is that the only way to change the driver is to delete, re-add, re-share the printer, and then add the updated driver. Though I really believe Microsoft must have provided some simpler way.