Tried it on my A1200. Can read the prop, set it, but the camera ignores what I set. Just like you.
I'll play with it a bit more.
Incidentally, the line at the top of your script that says :
props=require("propcase")lets you reference the various propcase values by name like this:
The available strings are on your SD card in the A/CHDK./LUALIB/GEN folder.
This has the advantage of letting your script work without modification on cameras that use different propsets.