Hi Rharbeg,
Welcome to forums
Open share point library and right click on form1 you want to edit.
Select Edit properties and change the name from form1 to Expense12345
You can give file name depending on the input of the filed.
Please look at that link given below for more information.
http://www.infopathdev.com/forums/topic.asp?TOPIC_ID=2455
Hope this will help you.