Set-GlobalAddressList In Exchange 2010 not modifying Custom GAL

We have several companies under our parent, each requiring separate GAL's in addition to the Default Global Address List. I have been able to create the additional GAL's via Exchange Management Shell using the syntax New-GlobalAddressList -Name "Brian Harris BMW Mini" -ConditionalCompany "Brian Harris BMW Mini" -ConditionalCustomAttribute15 "DistGroupBMWMini" -IncludedRecipients "AllRecipients". This syntax correctly created the Custom GAL, however we have had to make a change and only want to filter by the -ConditionalCustomAttribute15 value. I have used then following to attempt to modify the GAL Set-GlobalAddressList -Identity "Brian Harris BMW Mini" -ConditionalCustomAttribute15 "DistGroupBMWMini". Upon execution of the script I receive no errors, and the script completes and returns to the PowerShell Prompt. I then update the GAL with Update-GlobalAddressList command, and then use the Get-GlobalAddressList -Identity "Brian Harris BMW Mini" | FL to return the results. Long story short the -ConditionalCompany parameter is not being removed. HELP!!! Any ideas and suggestions are welcome.