Well, don't swinging and polyamory both come under the umbrella of non-monogamy, even though they're very different?
As for the word "lifestyle," I provided a link in my first post to a thread where we discussed how misleading the term is. The word "lifestyle" in my first sentence leads here:
http://www.polyamory.com/forum/showthread.php?t=9113
Swinging is a type of non-monogamous sexual activity, but people who swing can lead a variety of different kinds of lifestyles, can't they? Swingers call it a lifestyle, but that is just a way of coining their own terminology and a bit of laziness with the English language.
Polyamory is a way of cultivating multiple loving relationships into one's life, but there is
no one lifestyle that represents poly. So many ways to live polyamorously! There could be three, four, five, or many more people involved with each other, of any orientation, gender, what-have-you, living together or not. Poly people can be conservative and religious, liberal hippies and anything in between and beyond. There are numerous types of lifestyles people lead in which these choices can be incorporated, so there is no ONE poly lifestyle. I would say there can be no one swinging lifestyle, although there seem to be practices that are widely shared and understood.
If someone came here and said, "Tell me how to have a poly lifestyle," there is no way to answer that. They would have to look at how they live already and how they could fit polyamory into that.
To get back to your original question, then, isn't it obvious that polyamory is something people can have in their lives without ever swinging first? There are people who consider themselves polyamorous who have non-sexual partners, in fact. If someone just wants to expand and explore loving or being intimate with more than one partner, they certainly don't need to try swinging to get there!