Not everybody in the 'My Little Pony: Friendship is Magic' fandom likes the changelings, personally don't have an issue with them apart from when there are personality changes that take a while to adjust to. Not everybody likes Thorax either, again he's not a problem for me and actually like that he is a sympathetic character. Trixie though leaves me mixed, didn't like her to begin with but she grew on me. Really appreciated that "To Change a Changeling" explored a quite bold topic for 'My Little Pony: Friendship is Magic'.
"To Change a Changeling" does a quite good job with this topic, with a non-sugar-coated tone and a moral that is one of the most thoughtful ones of the season and of the latter seasons. It is not a perfect episode and is not one of the best of Season 7 (a season that started reasonably strongly but became variable post-"The Perfect Pear", a much better season though than the previous one) or of the show, but to me it was difficult to not admire it.
For me, "To Change a Changeling" would have been a better episode without Trixie. She doesn't serve much point at all to the proceedings, other than being a character for Starlight to spar against, and she is very annoying here due to her ego getting too big for her boots (or should that be hooves?). There are amusing exchanges between her and Starlight, and we do know from their previous appearances that their chemistry gels very well and that's proved again, but some of it is forced as well.
Really disliked Pharynx to begin with as well, even for a character you are not meant to like his jerkish-ness and arrogance were overkill and his inability to listen is frustrating. Actually loved what was done with the changelings, but their personalities here will take getting used to by fans that are more familiar with other characteristics. The method that Thorax managed to get them to change for a character as shy as him so quickly just like that needs some suspension of disbelief.
However, there is a lot to like here about "To Change a Changeling" despite how the above sounded. It is beautifully animated, the colours being eye popping and the backgrounds do so well immersing one into the world that is depicted here. There are fans that don't like Thorax's character design, personally don't have a problem with it. The music is dynamic with the action and has a good deal of atmosphere. The voice acting is very good all round.
Writing has some playfulness but what was more impressive was how it addressed a bold and difficult topic, that can easily be brushed under the rug, and deals with it in an honest and really quite gutsy way. The moral was very easy to appreciate and relate to, it's a very mature moral and one of the show's most realistic in a long time and is delivered sincerely and with punch. The story has tension and emotion without being overly serious, it was great to see that Pharynx actually did have some heart underneath that arrogance (unlike Prince Rutherford in "Not Asking for Trouble") and Thorax is as amiably sympathetic as ever with a touch of assertiveness. Also liked that the episode provides a reason for his shyness and it connected with me having been there myself.
In conclusion, despite some problems there was for me a lot to appreciate about this episode and it was one of the surprisingly good ones of the season. 7/10.