I'm totally new her and only been in the hobby for about 10 month (so please be gentle if I am totally off base here). But I believe most fish stores have central filtering systems, that filters and exchanges the water for all their tanks. So based on THAT - I would assume the same water/filter could be used for the new fish once the old were removed (I assume this would be a rather quick change so your BB won't die off).
This is just logical in my mind - but like I said, I'm still a rookie. Hope I'm not steering you in the wrong direction.
(EDIT: I just noticed you said you BOUGHT the tank with the fish already in it... So I guess it would also depend on how well you trusted the previous tank owner to maintain a healthy environment. I obtained some filters, substrate and a decoration from someone I didn't know for use in my new 75 - that will be up and running soon - but I cleaned the crap out of that stuff because I didn't know what kind of a tank it came out of)