Thank you very much Aditya, now it works.
As you said, the trick was to use \doiftextelse{}{}{} function as test of
emptyness function, and \labeltext{chapter} instead of
\currentstructurelabel as variable for the test because the latter was
never found empty, even when it was.
Thus the right code for me is:
\define[1]
\MyConversion{\doifelse{\currentsectioncountervalue}{1}{\doiftextelse{\labeltext{chapter}}{ONE}{#1}}{#1}}
\defineconversion[MyConversion][\MyConversion]
\setuphead[chapter][conversion=MyConversion]
\starttext
\setuplabeltext[chapter=]
\chapter{Cool}
\chapter{Cat}
\chapter{Mouse}
\setuplabeltext[chapter=CHAPTER~]
\setupheadnumber[chapter][0]
\chapter{Cool}
\chapter{Cat}
\chapter{Mouse}
\stoptext
___________________________________________________________________________________
If your question is of interest to others as well, please add an entry to the
Wiki!
maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage : http://www.pragma-ade.nl / http://tex.aanhet.net
archive : http://foundry.supelec.fr/projects/contextrev/
wiki : http://contextgarden.net
___________________________________________________________________________________
