The woofer and tweeter are direct replacements, and they can be exchanged through the front of the baffle. The cabinet stays the same.
There are some crossover changed required, but they are inexpensive and pretty easy to do. Some resistor values are different for the DE250 because it has slightly higher voltage sensitivity than the PSD2002. You can see the components used for each option in the "Notes" section of the crossover document.
Here are some of my thoughts about the upgrades available:
Your note at the bottom states that "Measurements were taken at a distance of 10 feet from the baffle and drive voltage was 8.62vrms. This is equivalent to 2.83v/M and 1W/1M values since average impedance is 8Ω."
Interpreting the graph, it looks like the 4 Pi speakers clock in around 98-99 dB/W/1M. This is incredibly efficient!
This should be able to provide ample power to drive these speakers. A quick calculation taking into account 99 dB/W/1M efficient speakers and 60 Watts of Power at 8 feet listening distance in a room with speakers positioned ~2 ft from the walls gives a an estimated SPL of 115 dB. This is more than enough.
If you search here for "flanking sub crossover" you'll find lots of threads where we discuss different crossovers and amps. Some are plate amps and some are separate crossovers and amps. But the truth is, you're right, that most plate amps offer only a fourth-order slope.
One of the exceptions is miniDSP. Not only do they provide stand-alone crossovers but they also offer plate amps. So that makes it really easy - just set it for "100Hz BW2" and you're done. Naturally, the subs need 10dB more gain than the mains too, which can be done either by attenuating the mains or boosting the subs.
Don't make 'em loud enough that you can hear them distinctly - They should be very subtle. You should be able to turn on or off the subs and notice very little difference except for more bass extension. The lower midrange will be smoother too, but that's subtle. So basically, the SPL should be set so the subs aren't noticeable. Each side becomes like a three-way speaker that sounds like a single seamless sound source.