You won't believe what they're replacing it with.
Entertainment Weekly reached out to Ciro Nieli, executive producer of the Nickelodeon reboot. According to him:
“There was a lot of talk about what the new ‘cowabunga’ was, or whether it should even remain ‘cowabunga.' We hadn’t initially come up with anything yet because we didn’t really necessarily need it [at the time]. But then when we were at the record, one of the actors — Greg Cipes, who actually plays Michelangelo — just kind of went for it. When he said it in the room, there were a lot of executives there. I think it was our first record. And it was that moment when he said it — I think he just really pulled it off and kind of won everybody over in the room. And it stuck.”