I added a custom attribute, Performance Measures of type string which
will hold enumeration values in an updated editor presentation -
Kind: Checkbox Multi-Select List; Attribute: Performance Measures;
Key: enumeration - Value: performance_measures.

When I create a new Work Item, the new Performance Measures appear as
checkboxes. However, existing work items do not show the updated
editor presentation. Why is that? Can I apply it to the existing
work items?

I added a custom attribute, Performance Measures of type string which
will hold enumeration values in an updated editor presentation -
Kind: Checkbox Multi-Select List; Attribute: Performance Measures;
Key: enumeration - Value: performance_measures.

When I create a new Work Item, the new Performance Measures appear as
checkboxes. However, existing work items do not show the updated
editor presentation. Why is that? Can I apply it to the existing
work items?

You can upgrade existing work items (the feature is admittedly very
hidden
at the moment):
1) Select the work items in the "Work Items" view
2) Right-click in the 'Type' column
3) Select 'Synchronize Attributes' from the context menu