I see. But as I said before: you could add the new voicelines, and from settings/options, let the player choose between the original voicelines and the new ones.
Besides, there's no need for the new voicelines to sound similar nor natural, since the original voicelines sounds very unnatural/robotic by themself. They just need the new ones to fit with 90s and Faith's atmosphere; I chose the VOX announcer as a suggestion for that reason, and I think Valve is reasonable enough to give you permission if you ask them (I can accept other alternatives, thought).