Here are the steps taken in the labyrinth which is to simply convert IDF versions:

i) Have downloaded some IDF files here https://figshare.com/articles/Fileset...
ii) confirm they are version 9.0.1
iii) Running 9.1.0 in windows 10 fresh install of Eplus asks for version updater. Opened the documentation auxiliaryprograms.pdf and no direct solution is given.
iv) Opening IDFVersionUpdater and browsing to file, it does not allow to convert because supposedly we have to download the library from help web.
v) Went to the said website and found only this: http://energyplus.helpserve.com/Knowl... which basically has incoherent text file with its contents adding nothing.
vi) Checked also the exe in C:\EnergyPlusV9-1-0\PreProcess\IDFVersionUpdater\Transition-V9-0-0-to-V9-1-0.exe (no 9.0.1 !?!) and ran as admin
vii) it opens a CLI window where asks for the path, after add the full path added click ok
viii) it starts processing, but apparently no file is created, and it simply proceeds to ask again for file path.

Thanks in advance for information, and btw for developers you should raise an issue to make these updates embedded, because documentation and website are obscure in my user opinion.

Regards.