I have the same problem. I see ARM, but the website says ARM64. I can set it to ARM64 in the settings, but after that, even the netinstall updates don't apply. After a reboot, everything returns. The command is accepted without errors.
/system/routerboard/settings/set preferred-architecture=arm64
And then I see ARM64
/system/resource/print but after reboot ARM I wrote to support. I haven't received a response yet.
In a nutshell, it is possible in theory, but in practice only the good Mikrotik guys can make it happen (if the rest of the features of the device allow 64 bit mode).
The main problem is that the website says ARM64 everywhere. The files for downloading are ARM64. The brochure on the website also points to ARM64. But the device already has ARM64. Then either the website needs to be corrected or the transition mechanism needs to be described. That would be fair.