Changed mine years ago, the only thing I replaced was the orifice hose,
which was why I took it apart and the O-rings on the compressor. The
orifice is crimped into a length of steel line, and the steel part
in-between the hose has a foamy insulator. Obviously condensation
collects in the foam and starts the steel tube to rust and pit. It
developed a leak. I also changed the drier.

Whjen converting Ac systems was in it's infancy, the engineers were worried
that older systems couldn't handle the change and had us change a lot of
parts - as it turns out, not with any real necessity.

The system should be drawn into a deep vacuum and as much oil removed from
the system as possible. The suction accumulator or receiver/drier should be
removed to get what oil we can out of them (but that's me being nearly as
anal as the engineers) - of course, we would replace the O-rings at this
time. I would like to R&I the compressor and remove what oil I could at the
same time.

Once we are this far, we can add the appropriate amounts of oil  (memory
thing coming.... about 3 ounces in the compressor... a couple of ounces in
the receiver /drier or suction accumulator... an ounce or so in each of the
condenser and evaporator and charge the system....

Here's the rub... many owners have had good luck simply doing a refrigerant
drop.... This is more luck than design IMHO... If you are looking for years
of reliable service without a compressor failure changing your budget, the
more we do when changing over, the better our chances.

YMMV

If you want to avoid the incompatible oil issues and potential
reduction in cooling capacity don't use R134 but get Freeze-12
instead.  If your system didn't have any real problem but you just
want to convert all you'll need to do is pull a vacuum for an hour or
so and then put the Freeze-12 in.  I just put the system in a 64
T-bird back in order and used Freeze-12 and it was putting out 45
degree air today in Phoenix.