Can you replace freeze plugs without removing the engine block?

Depends on the location, but usually. Expect the task to be very difficult. Some freeze plugs are behind motor mounts or in front of engine cradles. Some freeze plugs are behind the flywheel, which requires removing the transmission for access. Removing the freeze plug is only one part of the task - installation is as difficult as removal, if not more so. The freeze plug must be installed properly aligned or it will leak. Freeze plugs are interference fit, so some means of applying pressure to the freeze plug must be found. Usually this means putting a socket inside the freeze plug or on the lip of the freeze plug and hammering it in. This requires room to swing the hammer. Don't forget to place the freeze plugs in a freezer for several hours before installation. This shrinks the freeze plug a tiny amount, and every bit helps.