I would like to change the value of a property if it is a certain value. In C# I would write:

if(x=="NotAllowed")
  x="CorrectedValue;

This is what I have so far, please don't laugh:

 <PropertyGroup>
    <BranchName>BranchNameNotSet</BranchName>
  </PropertyGroup>

///Other targets set BranchName

 <Target Name="CheckPropertiesHaveBeenSet">
    <Error Condition="$(BranchName)==BranchNameNotSet" Text="Something has gone wrong.. branch name not entered"/>
      <When Condition="$(BranchName)==master">
        <PropertyGroup>
          <BranchName>MasterBranch</BranchName>
        </PropertyGroup>
      </When>
  </Target>