In article ,
Mxsmanic wrote:

If I am instrument rated and my aircraft is fully equipped for
instrument flight, and I start out a flight with a VFR flight plan,
and it gets foggy enough that I'm below minimums, is there a way to
switch to IFR in flight,

Yes. It's called a pop-up clearance.

or do I have to have filed IFR from the beginning?

No.

If it's possible in flight, how does it work?

Just ask ATC for a clearance. If they aren't too busy they will just
give you one. If they are busy you will need to contact flight service
by radio to file an IFR flight plan, then get back in touch with ATC to
activate it.

Landing somewhere first is also an option.

Is it possible to later switch back again?

Yes, you can switch from IFR to VFR at any time by simply telling ATC
that you are "canceling IFR."

I ask because it seems like it might be practical to start out as VFR
but still have the option of going IFR if the weather turns
unexpectedly poor (at least in terms of visibility or clouds). At the
same time, filing IFR for the whole flight just because part of it
_might_ be in IMC seems like it could be overkill.

Why? People fly IFR in VFR conditions all the time.

This of course assumes that a pilot and aircraft are ready and able to
handle IMC if they encounter it.

It's much better to be IFR the whole way if there's any chance of
encountering instrument conditions anywhere on the flight.

rg