What I really want to do (since I have 2 big honkin' oak trees in the
front) is to replace the centipede with st augustine since much of the
front yard is shaded.

How should I go about doing this? I was told by many that you cannot
get st augustine seed in north america, only plugs and sod.

It's not just in North America, I don't believe, there isn't a really
economically viable way to harvest St. Augustine seed, from what I recall.

If I sod, do I have to rent a sod cutter and take out everything? If I
do plugs, can I just plug everywhere throughout the lawn amidst the
centipede grass?

There are various ways you could go. On the less labor-intensive side,
spray the grass with roundup, let it die (including the roots). Once it's
dead, set your mower as low as it will go, and get rid of all of the dead
stuff. Apply new seed/plugs/sod, water, and wait. The new grass will
grow, and everything will be fine. Aerating the soil probably wouldn't
hurt.

Now, if you really do feel like putting in additional labor, you could
either till the dead stuff under, or use a sod cutter to remove it. If
you till, then you have to go back and smooth and level. If you remove
with a sod cutter, you need to cart all of that away. Either way, it's a
lot of work.

steve