It's difficult to say exactly what "Robot's Return" was like as it's only been described second hand, but in it a huge space vessel from other solar system beings humanoid robots who are following an ancient map in an attempt to discover their origins. They basically mistake Dylan Hunt for God, so it's "Changeling"-like in that aspect.
That's inaccurate. Here's the description of "Robots Return" [sic] from an old Lincoln Enterprises catalog I still have (undated, but probably from 1983 or so, since it only covers material up to TWOK):
The advanced computer and sophisticated machinery left on a moon of Jupiter by a 1992 NASA expedition have evolved into a new form of robot life and visit Earth in search of the "GOD" which created their life. They meet Dylan Hunt, formerly of NASA and consider him a messiah.
So the "huge space vessel from another solar system" angle was added later for "In Thy Image."
As for the "Changeling" similarities, they're tenuous. Nomad wasn't an evolved AI species, but a single individual created by an accident. And while Nomad was looking for its creator, there was no "God" angle to it, since 1960s TV censorship would've probably discouraged that. I suppose it's possible that Roddenberry was thinking in terms of getting to explore the religious angle he wasn't able to explore with "The Changeling," but conjecture isn't proof.
The mention of Jupiter suggests that Roddenberry was inspired by
Pioneer 10, the first space probe to fly by Jupiter in 1973, the same year
Genesis II came out (it was launched in 1972). With a NASA robot probe to Jupiter in the news at the time, and with Roddenberry developing a show whose hero was a NASA scientist, it made sense that he would've looked at
Pioneer 10, thought "How do I get a story out of that?", and come up with the idea of a later, more advanced robot probe to Jupiter (or rather, a computer system left there by human explorers) developing into androids that would come back to Earth. Now, that part of the story was dictated by the show's Earthbound format -- Dylan couldn't go to them, so they had to come to him. And since the idea would've been to tie into his NASA history, having the androids descended from NASA tech latch onto him as their "god" was a natural outgrowth of the premise. It's conceivable that "The Changeling" was in the back of Roddenberry's mind when he thought up that aspect of the story, but given the Jupiter focus and the timing, it seems logical that
Pioneer 10 was his primary inspiration.
After all, this is a time when Roddenberry was trying to grow beyond
Star Trek, to create the
next big thing he could use to grow his career and his reputation. His hope for
Genesis II and the other series premises he was developing at the time was that one or more of them would be a greater success than
Star Trek was. After all, at the time,
Star Trek was just a show that got cancelled after 3 years and was never likely to come back except as a cartoon. So he wouldn't have just been trying to tell stories that reminded people of that old, cancelled show. He would've been looking for new stories to tell, stories inspired by current events and new scientific advances like Jupiter probes. Stories that tackled themes he wasn't allowed to tackle in the '60s, like religion. If "The Changeling" entered his thoughts at all, it was probably as an example of something to surpass.