If you have less number of Quant then you can use transaction LS22-Change Quant to do it manually one by one or else you can go for direct table update if it is allowed in your project (for mass change) of SLED (Not advisable but i think you don't have other option available here for mass change)
by updating LQUA you can get the required detail in future transaction from WM perspective however if you have any custom report which picks the data from the Delivery table or material document table then you need to update those table also
MSEG-VFDAT
LIPS-VFDAT
Message was edited by: Manish Kumar