How can we detach a reference file in a drawing and add a different one thru Projectwise without opening an autocad session ?
Detach a file and add a new reference file thru Projectwise.
You can't. You can replace the existing projectwise reference with another file in projectwise, but you can't detach and then attach a new reference without opening AutoCAD (or MicroStation) to do it.
To replace a reference file in Projectwise:
If you want to retain a copy of the existing reference first version before doing the above by by right-clicking the reference, select New > Version. After creating the version do the steps above on the master version of the reference.