Maybe I didn't quite understand your question...
When in the below practical example description of events during a non-precision approach would you propose:
to update the CRZ level on the FMC
Practical example:
Controller vectors you towards final approach course of VOR approach which you have elected to fly as a VNAV Approach.
You select the altitude he clears you to in MCP (you may still be above published platform altitude as he may vector a straight in rather then with a procedural turn for which the platform is used).
You arm LNAV to capture inbound track while VNAV remains engage and will start slowing the aircraft down in good time.
Using the CDA capability VNAV will automatically transition from Idle Decent to Approach Logic, allowing Flap Extension and a continued CDA onto Approach.
Check your FCOM and see when VNAV starts its Approach Mode.
You may find that your actions only make sense if in a descent.
VNAV Approach Logic should be defined in the VNAV literature for your type.