Is it possible to switch from VFR to IFR and back?

writes:

I get a weather
update from them, and if there is so much as a hint of clouds in the
Bay Area, I file
and IFR flight plan with them, beginning at the next waypoint on my GPS
flight plan.

By this you mean that you essentially just read off the waypoints on
your GPS to them as your routing?

About 10 miles from my waypoint, I mention to ATC that I would like
to activate my IFR flight plan starting at ***. They give me the new
clearance, I write it down and immediately turn to the new heading
and altitude. Poof, I'm IFR.

Sounds great.

So do you plan waypoints and fly them for all your flights, even VFR?
If so, I imagine that makes it a lot easier to switch to IFR if the
weather doesn't cooperate.

I usually fly that way in the sim, too, and even when I'm being
"casual," I keep an eye on nearby radio fixes and other things I can
use to find my way if the weather changes. If I'm set to use
real-world weather, it often _does_ change, and then I just switch to
instrument flight.

Which makes me think of something else I was wondering: How do you
work things if you are on the ground at the airport, and there's a
layer of fog or clouds that prevents you from taking off under VFR,
but you know that the weather is clear only a few thousand feet above?
Do you file IFR and fly it until you are in clear air and then cancel
the IFR and switch to VFR, or what?

Does ATC get mad if you switch back and forth to and from IFR as
conditions change?

One common example is - you're flying along over a cloud
layer and you'd like to land the airplane. So you ask ATC for
an approach. I get the feeling that it's easier to get a single
approach out of ATC directly than a whole flight plan.

That's one scenario I've been wondering about. So do you just call
them and say you'd like to switch to IFR and ask for vectors for your
approach, or what? How much formality does switching to IFR flight
require?

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