If you have an external reference for the part you want to replace, it should be able to work via the management of external references (when opening the file for example).
In the editing of references to the opening of your multibody part, there is the 'replace' tool at the top right of the window that allows you to replace the name or path of your file with another one. Double-clicking on the file or path name also works.
I tried with the external references, I can replace the reference with the one I want, but after opening the file, nothing changes.
I tried by tree copy and Pack & Go, but it's the same thing. The link does not change.
There is however a way to change this I guess, because as you say froussel, there is an external reference to this imported body. By the way , via EDIT IN CONTEXT when you select the body, you open a room ...
Weird that it doesn't work. We work a lot with inserted parts (raw from the foundry inserted in the finished part) and we have never had any problems changing an external reference. Maybe because you have several external references in the same room?
I just did the test with 2 pieces. In practice it works but not terribly.
When you edit your reference in the reference edit menu (the line then turns green in this menu) and open your file, you have the impression that nothing has happened: you see the old 3D instead of the new one for the body you edited. Masi SW did the replacement well: just "edit in context" the body you modified. SW then opens the new file and when it closes it updates the 3D in your main room.
Big Note : there is a SW system option that handles this behavior: option/ external references/ load referenced documents. If it is set to 'none' the behavior described above is normal. We try to be on 'modified only' (should be enough for your case) or 'all' but you should know that this SW parameter is not stable at all in SW. I reported the problem to visiativ under maintenance (their answer: The windows registry key is Computer\HKEY_CURRENT_USER\Software\SolidWorks\SOLIDWORKS 2020\General\DontAskAgainOptions\MSG_LOAD_EXTERNAL_DOC%*^%$&23112 when the option is in "On request" the key is deleted). For Visiativ you would have to load the SW parameters at each boot to make sure that the values of this key do not jump randomly (which is our case now).
If you set the setting to 'all', the file updates automatically every time it is opened. On 'modified only' it should work when you change the reference. I strongly advise you to set to 'all' to make sure that the references of your model are always up to date (otherwise you may miss an imported part modification).
A bit destabilizing it is true (NB: I am on SW2020 SP3.0, it is possible that the operation is different on other versions)