I might be thinking too deeply into this, but perhaps this song might mean something else as well, not just for Thorax, but Spike could also be singing about himself.
He was talking about that he could sometimes make mistakes in his life, and that he doesn't feel so heroic. That could reference Spike's past episodes before this season, about he used to always mess things up, and sometimes his emotions get the better of him, and he could be selfish at times, but when he says "A changeling can change", it could also be a metaphor, saying he could learn from everything from his past episodes, and change into a better him!
When he says "this changeling it seems, knows the real me", maybe it could mean there are people in the fandom that actually see the good of him, and not just his comic-relief butt-monkey role, and we'll "stay by (his) side until the end".
Like i said, I might be thinking too deeply into this, and I don't think it was the intention of the writers to be displaying this message, but that's what I see in the song. Spike looked like he was singing about the changeling, but he could also be singing about his role in MLP, and how it changed over the years!