1- PCS7 supports S7 300s and S7400s. You can view the 'PCS 7 - Released Modules' documentation for each version of PCS7 on the support website.
2- I am sure of the question you are asking. You wrote a program in STEP7 5.5 (is this what you mean?) and can you clarify what program you used to write (example..SCL)? PCS7 allows for ladder, SCL, FBs.. etc. But you may want to educate yourself with the PCS7 Libraries, as they may have also solved what you wrote, thus you can keep with a standard approach and not run into issues during upgrades.
3- Again, here I am unsure what the question is asking. Are you asking if all the programs need to be programmed in CFC? STEP7 programs are used in PCS7 so if lots of your program is just ladder logic, and DB and FB manipulations, yes it would work too.
But the thing to think about with PCS7 is the whole platform. Consider your HMI.... valves, motors, PIDs, all these parts are conquered already with the Standard libraries in CFC and create tags and icons in the HMI. I would imagine you would want to replace as much 'custom' as possible with standard.
Hopefully I have answered you questions.
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