New enumeration field can't be filled in old existing WI

Hello,

When I add a new enumeration drop-down list field in a RTC workitem, this field is working properly only for new workitems.
Older workitems (previously created) have this field present but totally greyed and can't be filled !
How can I modify value of this field for old existing workitems ?

I'm currently using RTC 2.0.0.2 iFix3 with Oracle 10G DB.
And, for information, I have strictly the same behaviour with RQM 2.0.1 iFix3 workitems.

Does anyone have encountered this problem and have a solution or a workaround ?

Thanks.
Regards,
Jrmie