Is it possible to update an customAttribute ID which exist ?

In RTC 3.0 I try to customize the risk workitem (extract of Formal Project Management process).

In the implementation of risk, for display correctly the risk matrix, we need used this Attribute ID with a type = "probability" (enumeration id)
<customAttribute id="com.ibm.team.workitem.workItemType.risk.probability" name="Probability" type="probability"/>

But in my project area, i already have this attribute id but with type = "hmlScale" like enumeration id.

<customAttribute id="com.ibm.team.workitem.workItemType.risk.probability" name="Probability" type="hmlScale"/>

I try to update the hmlScale but the risk matrix not display correctly