You've got a couple of options here. You can try and find a copy of the Windows 8.1 that doesn't have the install.esd or You can convert, like you mentioned. If you convert you've also got a couple of options as well:
If you convert, you can use the Windows 8.1 ADK and try and figure out how to do so. This is a fairly complicated process.
You can download something called WinReducer ES. Here is the link: http://www.winreducer.net/winreducer-es-wim-converter.html NOTE: Make sure you fully read the instructions and download what is required to make it run. This will not work if you try and convert in a mounted ISO. MAKE SURE YOU FULL READ the instructions! :)
It's not that sounds complex but it does sound like it can be frustrating if you don't!
I've never run into this particular problem. Thanks for letting me know that this is something that is happening when they download the 8.1 ISO.