Well, I've had to do that before. I had to veg. using my 6500k CFL bulbs.. (because I couldn't find a conversion bulb that would work with my HPS ballast).. and then flowered them using my HPS light.
So yeah.. you can do it that way. As for answering your question about if your buds will be dense.. that really just depends on a few things.
The strain your growing.. (some strains are just more dense than others are); Type of lights your using.. and so on.
But I will say this.. I had to do what your talking about during one of my grows.. and I liked the end result at harvest. The buds weren't really DENSE DENSE.. I mean they were'nt rock hard or anything.... but then again.. they were airy and light either. I was happy with the yield too.
So as far as that question goes.. it just depends.. and really.. you won't be able to predict the answer to that question. You'll just have to try it and see man.
I've learned.. (and other experience growers know this too).. I've learned that growing is pretty much a "Trial and Error" process some times.
Good luck man.
Hope that helped you out some..
peace.