I once had a similar problem and resorted to PowerShell:
Get-Content .\my_UTF8_file.txt | Set-Content -Encoding utf32 my_UTF32_file.txt
or
Get-Content .\my_UTF16_file.txt | Set-Content -Encoding utf32 my_UTF32_file.txt
Kudos to https://superuser.com/questions/1163753/converting-text-file-to-utf-8-on-windows-command-prompt
+ reference https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.management/set-content?view=powershell-7.4#-encoding