If you're using the CLI, for the ACNS 4.x to 5.x upgrade all you need is the .bin file. For the CDM-based procedure you need a .meta file and a .upg file to feed into the CDM GUI. The contents of the .upg file are the same as the contents of the .bin file. The naming convention is a historical difference arising from different code bases being used. For current ACNS releases I'm pretty sure we just release *.bin files and no *.upg files.
For 4.x to 5.x you don't need to use the *.sysimg file. That file is there for disaster recovery purposes only (e.g. to recover from a power failure during a flash write of a new image). If you install a version X.Y.Z-b *.bin file and then install the X.Y.Z-b (same build) *.sysimg file, the latter step has no real affect -- it's installing the same bits.
By the way, the cache 2.x to ACNS (4.x or 5.x) upgrade that you mention is more involved internally than an ACNS to ACNS upgrade. Mainly this is because ACNS 4.2.x and 5.x are internally fairly similar, whereas cache 2.x and ACNS (any version) are internally totally different. Also, the cache 2.x procedure requires the intermediate 2.x-UPGRADE image because the 2.x code base lacks the ability to write arbitrary new images to flash; the 2.x-UPGRADE image adds this ability. So, for 2.x to ACNS you really do need three separate files.
