Confused and unsuccessful - in place upgrade from Win 8 Ent VL to 8.1

As the title says - I'm trying to do an in-place upgrade of Win 8 VL Ent to Win 8.1 (or 8.1 Update 1).

Reasons for in-place don't matter but if curious: the situation isn't one where users will have issues if it's not clean installed, the hassle of reinstalling and reconfiguring to how they had it, and also they're keeping Kaspersky 2013 which is compatible after an 8.1 upgrade but blocked by the installer upon fresh install (go figure!).

They are in exactly the situation so many posts complained about with the upgrade process - 4 person site using Enterprise VL because of useful features, and no IT department. As a friend, I'm their "go-to" guy when they get stuck.

There are dozens of threads on this issue, most of them say an old-style in-place offline upgrade is possible using the correct 8.1 media, or after in-place "upgrading" to W8 non-VL. That's what I'd like to learn how to do. All the pages I read say it's possible but I've failed each attempt and spent a day so far trying to solve it. Tried so far:

1. Downloaded "en_windows_8.1_enterprise_with_update_x64_dvd_4065178.iso", checked hash, unpacked it and run setup.exe - says "no upgrade path".
2. Tried the suggestion in many threads of "setup /auto:upgrade" - says "unknown command-line option".
3. Tried the same with plain 8.1 rather than 8.1 update. Same.
4. A number of threads say to run the ISO off a USB stick or DVD, and not from an unpacked folder. AFAIK an in-place upgrade in Windows 8 has to be from within Windows, meaning the "/auto:upgrade" option will still be absent (doesn't magically appear just because the file is now on a USB stick, I guess). So I haven't tried this as it's not clear what I'd have to do differently, or if this needs some specific version of the install media I don't have.
5. Tried the hint at blog.williamhilsum.com/2014/03/how-to-upgrade-windows-8-volume-license.html (text link only) to downgrade from VL to retail using default keys, I get an error described by "slui.exe 0x2a 0xc004f069" (doesn't say much but that's what I got)
6. Tried to find a log of the exact issue. Setupact.log and setuperr.log don't show anything relevant except: "Failed to change the UI language to en-US" and "Failed to load en-US resources with error 2. Will load fallback binaries": I'm using en-GB here but both that and en-US are installed on the system? Not clear if this has anything to do with the problem though.

It would help immensely if someone could set out clearly, the exact steps needed to do an in-place offline upgrade from 8 Ent VL to 8.1 or 8.1 Update of the same edition. If I need some magical exact version of a Windows ISO which one(s)? I feel dumb for asking but I've found no thread that sets these steps out and has actually worked for me. It's clear that it's flummoxed a lot of people but also posts suggest that it should be straightforward given the right install media version, or the right magic....?