I haven't lived in Mississippi for 2+ years and I still vote there. This isn't an issue. 
I don't think it's voter fraud unless you're double-dipping. Simply for car registration and license reasons, though, I'd recommend making your relocation official. I procrastinated on that for a year after my interstate move and didn't encounter problems, but it easily could have been a worse inconvenience.
As to the OP, my state relocation was shortly before the 2016 presidential primaries, and upon researching that question, it definitely seemed harmless and legal to vote (in person) in my old state, when my residence in the new state wasn't yet official.
