Yeah, Park is a series regular this year. She isn't going anywhere. I bet the new character (who, currently, seems to be a needless addition*) has an extended guest run with the option to later be picked up as a regular (assuming, TPTB think the character works).
* Yes, I don't care for her just quite yet. I think the biggest problem I have is that she was haphazardly introduced in the show with a purpose that is very ill-defined and portrayed. Apparently the new governor wants her to provide some sort of oversight to the Five-O team. That's not a bad idea in itself as it can create some good conflict.
Where I have problems is that she didn't want the position let alone know that she was going to be assigned it until a minute beforehand. She then continued to protest it. What kind of good, honest oversight can she possibly provide for a position that she was dumped into that she didn't know existed nor want?
It seems like the writers were juggling with two concepts and just decided to smash them together.