What matters is the content of the pitch documents JMS showed to Paramount. If they used the word changeling, it's problematic for paramount.
No, it's not. It's a non-issue! Changeling is an existing word, no one owns it. The characters in question aren't even remotely similar. One is a minbari assassin who uses technology to assume another person's identity and frame them for murder, the other is a member of an unknown species with he natural ability to mimic other objects, animals and even people.
So what did they have in common, based on Emissary:
1. A "changeling"
2. The wormhole/jumpgate
3. The religiously oppressed race and the oppressors
4. Diplomatic aim of the station (Sisko was ordered by Picard to get Bajor ready for federation membership)
5. The haunted commander who survived a massacre
6. The multi-species, transit hub nature of the station
7. Angel-like aliens
8. The station situated in neutral space
9. The darker tone
10. The prejudiced, female XO (according to the B5 pitch bible)
2. One is a one of a kind stable passage to a fixed location on the other side of the galaxy and the reason the station becomes noteworthy, the other is just the regular means of interstellar travel and nothing special in universe.
3. Generic backstory and the portrayal is very different. The Gathering actually portrays the Narn as more aggressive and vengeful which DS9 quickly rejects for the bajorans.
4. Babylon 5 was created as an interstellar diplomatic and commercial hub to hopefully prevent another war. DS9 was an old ore refinery/administrative center that's repurposed as a commercial and travel hub after he discovery of the wormhole, the station itself had no diplomatic aim.
5. Kirk survived a massacre too ...
6. That's the purpose of most stations.
7. There are no angel-like aliens in Emissary, the prophets are portrayed ad extra dimensional aliens with no concept of linear time, there's nothing angelic about them. There aren't angel-like aliens in The Gathering either, the Vorlons aren't revealed to be angel-like in appearance until the season 2 finale.
8. DS9 isn't in neutral space, it's in bajoran space.
9. This is ridiculous
10. Takashima wasn't prejudiced, "according to the B5 pitch bible" isn't a valid argument as we haven't seen it.
Some of those are false equivalencies, some are superficial similarities that are vey different when looked at in detail.