Do not attempt to use an NCE 5 amp / 10 amp / Power Pro / PH-Pro / CS02 system to program CVs>256 in Program Track Mode using the Pro Cab. You will corrupt other CVs in the decoder.
Your options are:
1) Use the built in serial interface and a PC running JMRI / Decoder Pro. It knows how to work around the firmware bug in the Power Pro and it also takes care of setting the index CVs 31 & 32.
2) Use Program on Main but you MUST first manually set the correct values in both CVs 31 & 32 BEFORE changing any CV>256.
Depending in the type of NCE system you have AND the actual decoder, accessing and changing CVs above 256 might require a different method.
The 5amp PH-Pro has a firmware bug that prevents access to CVs above 256 on the program track ONLY. You can use POM - Programming On the Main with no issues.
The Power Cab has no such limitations.
Depending on the decoder might need to deal with indexing too:
To access CVs above 256 you need to set one or two “indexing” CVs first.
Read this: https://dccwiki.com/Configuration_Variable#Indexed_CVs
Thanks to NCE "Super User" Dave Heap for this information.