Could a invertebrate, for this lets say, a centipede/millipede, evolve into something like that of Pikaia? or something similar? Just an invertebrate turning into something similar or what would look like a vertebrate. Is that even possible in the first place? I was thinking maybe tagmosis could do some of the magic but other than that, I hit rock bottom.
This is a very complicated question to answer. evolution of that extent might take many millions of years and nobody can say for sure whether something like that would actually happen.
If we look back in time (https://socratic.org/questions/how-did-vertebrates-evolve), organisms with exoskeletons such as ostracoderms appeared before vertebrates. So, perhaps arthropods like centipedes and millipedes could evolve into a vertebrate if it benefits the species in their particular environment.