Hi Jen,
In your situation the thing I think you need to do is NOT delete the actual node in the form schema. If you remove a node or field in the form it will break old forms as you are changing the schema.
Removing the field from the UI of the form is not the same as deleting the node in the data source in InfoPath.
So for example in the attached screenshot I have a field called "CustomerChild" that I have been collecting data with and have 100 forms that have that data. I no longer want to collect that data. What I can do is simply remove the field from the UI but not the Schema. Old forms won't be broken. The data will still be in the XML of the form for old forms.
If you want to get really fancy you can do custom code for upgrading the form. InfoPath has some built in functionality to support upgrades but sometimes you do need to write custom code to upgrade forms correctly. If you search a bit in the forums I am sure there are some posts or blogs on doing custom upgrades in InfoPath if you really need that.
Ted