With replace option an Assembly disappears after searched by Where Used in structure editor
Solution This is working as designed.
Top-level assembly file replace is nothing but opening a new file.
This is not a bom line (Assembly).
Replace should be used as follows:
1.Open an assembly
2.select one of child
3.replace it with other child component.
Notes and References IR 9142640
Hardware/Software Configuration Platform: INTL64
OS Version: 764