This is interesting. When I was trying to make my mono boyfriend understand polyamory, I was telling him that one person doesn't replace another, that every person is different, and that you express yourself in different ways with different people. And he said something like, 'Yeah, but you have your friends for that'. So, in the 'traditional' point of view, you can (and are encouraged to) non-sexually express yourself in different ways with different people, but you can't do it sexually. Then again, the boundary between the two scenarios may be as blurry as you can imagine, right?
Make new lovers, but keep the old.
Those are silver, these are gold.