How to sort and display a field after it process through a when choose statement?

I have a need to apply a when choose condition to a field, Party_name.  The field party_name is display as “Last Name, First Name” and it is in the Last name order.  After through the “when choose” statement, the field Party_name should display as “First Name  Last Name”.   My “when choose” statement is working but the Party_name field is still display as Last Name order.  How can I sort the party name (after when choose) in First Name order.   Any helps are greatly appreciated.

Here is my Syntax on display party_name as “First name Last Name”