Hello
What I understood is that after the restore of the database to the target system you will do a local client copy from 300 to 200 and then delete client 300. Correct me If I'm wrong.
BW objects are cross-client. So when you delete the client 300 you are most likely to delete the complete data.
In addition to the above notes read these SAP notes.
Note 793717 - Client copy into the BW client in Netweaver/mySAP ERP
Note 522569 - BW: Working in several clients (especially APO / SCM)
I would get hold of a BW Consultant to know the possible issues they can face after this activity and then consider whether it is worth changing the client after the restore (system copy).
Good Luck
RB