\convertnumber does not work on command parameter. What am I doing wrong? I know that I can use conversion key in setuphead, but I need slightly more complicated command.

Use \setuphead[section][conversion=I], you can’t use \convertnumber

because the first argument is the formatted (styles and colors are applied)

content of the section number.

Thanks, I'm aware of this method and I mentioned it in the original question. The question is what to do if I need to use the command key to apply more complex formating using \framed etc.
At least I need to make  \section{My section title} to print

I. Section
My section title