Can't delete any geometry in a migrated part file.
When a migrated part file is opened in NX there may be geometry in Modeling that can be selected and modified but can't be deleted.
Solution
1. Verify the file has been migrated by selecting File-> Properties and there will be a 'Migrated Item' tab.
2. Once verified select the Reuse Library tab on the Resource bar.
3. Expand Custom Symbol Library-> Part Symbols.
4. Delete the Migrated Symbols folder.
5. This will delete the unwanted migrated geometry.
Notes and References
Hardware/Software Configuration
Platform: all
OS: n/a
OS Version: n/a
Product: NX
Application: GATEWAY
Version: V1892.4100
Function: ATTRIBUTES
Ref: 001-9968189