Hi Tong,
If nothing has been changed in DEV after installation then backup & copy should be fine. I was thinking that you want to move custom development that way; recommended way for it is Transport system.
Not sure if this will be faster than just installing everything in QAS.
I'd think that the only follow-up activities would be change of references: to itself and to other systems. If active BW is in same box then it's more to change; if BW only serves as a platform for BPC then it's only SM59 and SPRO.
Regards,
Gersh