Changing that one word might effectively do away with the law. Anyone might be able to claim that the arrest is unlawful and have it stick. If we get enough charges of unlawful arrest tossed out because the suspect believes the arrest to be unlawful, the law could become more trouble than it's worth for the state to prosecute. It's not that I have a problem with that, I have a suspicion that many charges of resisting arrest are bogus to begin with, it's just an observation.
This change in the law is very much needed. We cannot allow an officer to execute an unlawful arrest but still be able to prosecute a citizen for resisting said unlawful arrest. This law, as it stands now, allows for the police to abuse their power of arrest to intimidate and punish people that might somehow figuratively rub an officer the wrong way.