If you use PC you can use a batch convert command from .cube to .cub (you need some DOS command skills to do this) https://www.c3scripts.com/tutorials/msdos/commands.html
Open CMD on you PC (https://en.wikipedia.org/wiki/Cmd.exe)
Click Startbutton and type CMD and press enter, the command prompt opens. (a small black window)
Go to the folder, using dos commands, where the .cube is located (I recommend you copy the .cube folder you use)
When you are in the right folder type
for /R %x in (*.cube) do ren "%x" *.cub (this will change the filename extension of all .cube files to .cub in that folder)
For example
C:\user\me\documents\luts>for /R %x in (*.cube) do ren "%x" *.cub
Basic DOS
type "d:", "e:" to change drive
type "cd foldernamn" to open folder
type "cd.." to go back to previous folder
https://www.c3scripts.com/tutorials/msdos/commands.html