Backports and upgrade to buster

[Georgios <gpdsbe+debian@mailbox.org>]

Hi there!
I'm thinking installing debian on a new laptop. The problem is that the
current version is stretch that comes with an old kernel that doesn't
support my wifi card.
I was thinking about using backports to install the latest kernel.

I have the following questions.

When I install stable and use a kernel from backport when you install it
you upgrade a couple of packages like libglib, firmware-linux-free etc

When the buster is released what happens when i try to upgrade the
system from stretch to buster?
Do I have to remove backports from sources.list?
What happens to the packages that were upgraded from backports? Are they
replaced by the new stable packages?

Thanks in advance for your help.

ps.I know we are close to the release of buster.I could simply choose to
install debian buster. But i would still have the same questions if I
use backports in buster.