I think typicaly when you buy an aftermarket BOV you do replace the old one. Also many people replace their stock one because they are kind of prone to leak a little oil over time (at least mine was and i have read of it happening to a lot of others).
The stock BOV will hold enough boost at BPU with no problems; I believe the only concern is flow rates.
I and a lot of other people mount an aftermarket BOV with the stock BOV in its stock location. This allows some air to be recirculated back into the MAF so that the engine doesn't temporarily run rich and stall.
Others just remove the stock BOV and mount an aftermarket BOV in its place or another location (after plugging up the stock mount holes). This may cause stalling on untuned MAF-equipped cars.