It would probably depend on how specific the spell was, if the spell was 'turn the pumpkin juice into alcoholic chocolate milk' then any Bleeprin that was also in the glass (or whatever container it was in) would remain unaffected, and react accordingly when it suddenly becomes immersed in alcohol.

If the spell was 'turn the contents of that glass into alcoholic chocolate milk' then any Bleeprin that was already in there would get transfigured at the same time as the pumpkin juice, and so wouldn't react.

I currently don't have my books, so I can't check this, but my impression of the magic in Harry Potter was always that the spells were very restrictive in what they did, and that any major changes (such as the starting form of an object to be transfigured) would require a different spell. That being the case, I would expect the spell you mentioned to specifically target 'pumpkin juice', so yeah, any Bleeprin that gets snuck in first would result in an explosion, although it probably wouldn't be that big.

As a note, I believe that 'Bleeprin' should always be capitalised.