A CG character's worldview has been shifted dramatically by his experiences in the Underdark.

If he begins to act in a manner that has a CN bent, will this be viewed negatively? Or is it the best (and only) way to shift his alignment?

I don't want to be viewed negatively for RPing a CG character that is changing his views for IC reasons.

His demeanor towards others is changing, his ideas about right and wrong are changing, and his goals are changing.

My basic question is: If a DM sees a character that began as CG acting in a CN manner, do they automatically assume inconsistency, or do they assume that there is an IC reason for doing so?

Thanks if you respond, and thanks for the awesome place to roleplay and play D&D.

If a chaotic good character is acting chaotic neutral, there is inconsistency. That doesn't mean we'll chastise you. If we're not sure why somebody's acting in a particular way, we may inquire with the player, but often we just apply alignment points as necessary. You should not pick the alignment you want to end up at during creation, you should pick the one that best describes your character's PoVs when you begin playing.

Coming to the Underdark and shifting from CG to CN in attitude and actions is entirely reasonable and A-OK with us. Welcome to the server!