Hello
I don't know of a miracle method (but I don't have all the CATIA modules).
To my knowledge, there is no CATIA command to isolate an instance from the original file.
So I would say that there can be 2 needs
1 Completely independent piece and instance
2 Piece and instances evolve simultaneously but have small particularities.
Case 1 :
1 Open only the room
2 Under the properties, edit " Reference " (otherwise CATIA will notify you when replaced).
3 Save As : (change file name)
4 Opening the Assembly
5 On the instance to be made independent (right-click/Replace component)
6 In the window that opens, uncheck (YES) to replace only the selected instance.
I don't know the project but replacing one part with another, even if it's the same with a different name is a different reference, will have an impact on the assembly.
First of all, on the constraints if the publication has not been used and especially if they are taken from the geometry and not from the construction elements of this geometry.
On possible copy/paste with link between parts.
On the 2D.
For Case 2, the room must have been structured for this, the common parts under the same body of parts, the specific parts under one or more others.
We copy the common part with link in a new part and then we will make the specific parts under other parts bodies after we replace the desired instance (Step 5 ; 5 ; 6).