In the Solid Edge ST9 Revision Manager it was possible to replace one Solid Edge file type with another in a Draft e.g. replace a Part with an Assembly. In Solid Edge 2020 it appears that the Part file can only be replaced with the same file type.
Solution
This is correct if the Replace functionality on Design Manager home tab is executed via a right mouse click pulldown menu on a file - then only the same file type is available for the replace.
If another file type is required to replace the existing file linked to the Draft then you need to use the 'Design Manager Assistant' which is available in the 'Tools' tab and will guide you through the necessary steps.
Notes and References
Hardware/Software Configuration
Platform: INTEL
OS: windows
OS Version: 1064
Product: SOLID_EDGE
Application: DOC_MANAGEMENT
Version: V220MP2
Function: DESIGN_MANAGER
Ref: 001-9633617