There is a very robust updating mechanism from inside OPNsense, which works reliable and also easily manages major version jumps. Furthermore the images for installation of OPNsense are indeed called "amd64" (see https://opnsense.org/download/) but this denotes the architecture and not the CPU type. Means AMD has introduced the 64bit extensions to the i386 architecture, this is why they are called "amd64" (see e.g., this explanation why 64-bits versions are called amd64). So in short: of course OPNsense images run also with Intel CPUs.