porcher reprise cartridge removal- diy or plumber?

[diymomma]

Shower going on a rampage- only hot water comes out. Porcher kindly sent a new thermostatic control cartridge. When I asked if it was something I could replace myself, I was told "depends on how handy you are." Fair enough. So-- I have the handles, cover plate, escutcheon and temperature cap off. I'm staring at the temperature calibration unit, and a diagram showing that behind the calibration unit is a handle extension before getting to the thermostatic cartridge. It is not obvious how these last two are removed, or what to do to remove and replace the cartridge. Can someone out there guide me on how to procede without causing more trouble than it is worth, or do I need to call in an actual plumber for this one?

thanks