I dont think Jerry Nelson ever actually took over the role (though I heard he performed the voice in a Camp Wanngohoma sketch).
One thing that I've noticed is that in the song Roosevelt Franklin's Alphabet, Roosevelt's voice is much deeper than usual, and Hard Head Henry Harris' voice actually sounds similar to that voice. I don't know if Roosevelt was just voiced by a different performer in that sketch or if Matt Robinson used a different voice at first. Roosevelt Franklin's Alphabet is from the first season, as is Roosevelt Franklin's Days of the Week, where Roosevelt has his more familiar voice. Of course, in the alphabet sketch, his voice sounds closer to Matt Robinsons natural voice (assuming that Matt Robinson talked in his own normal voice when he played Gordon) than it does in other skits. I guess it's possible that Matt Robinson wanted to do something closer to his vocie but then felt that Roosevelt should sound different from how Gordon sounded, or maybe the deep voice was too hard on Matt's throat.
Of course, if Matt Robinson did originally perform the voice of Hard Head Henry Harris, then I wonder why he would stop performing the voice of him but not Roosevelt. I've heard that Hard Head henry Harris sand The Twelve Days of Christmas on a Sesame Street christmas album. On that track, does he sound the way he sounded in the africa sketch? Do the album credits list Roscoe Orman as Hard Head Henry Harris?
It's also possible that Hard Head Henry Harris continued to appear after Roosevelt stopped appearing, and maybe Matt Robinson left because he couldn't be Roosevelt's voice anymore. But I have never heard of any sketches that have Hard Head Henry Harris, but not Roosevelt.
I would suspect that Hard Head Henry Harris was voiced by one of the human actors before Roscoe Orman took over. If I had to guess, I would guess Hal Miler, since he obviously left the show before Roscoe Orman took over, and I can't think of any other humans who were introduced in season four who left in season six. Unless it was another cast member who just gave up the role (maybe Will Lee?).