I think it would have been really cool if Raj and Underhill had been switched. Making the gay man who admires Alec and his work a MOC and the racist asshole underling the generic white dude would have worked a lot better to me.
Hm, to me that only works if the ages of the characters - and subsequently the actors - changed. Raymond Ablack’s Raj looks about the same age of Matt Daddario’s Alec and that works to show how a Shadowhunter that age is supposed to behave. Raj was there before Lindsay and Duncan were introduced and he was already fulfilling that narrative role.
Steve Byers’ Underhill, on the other hand, looks older than both of them and that’s important for the character of Underhill. He needs is to be a significantly older Shadowhunter from outside of Alec’s social circle. That’s the point of the character, especially because we will have Aline as the young and already known Shadowhunter inspired by Alec.
Now, if it was a younger looking white actor playing the character equivalent to Raj and an older looking desi actor playing Underhill, that works for me too. If it was up to me, though, I would keep Raymond Ablack’s Raj because I love that asshole and give Underhill to someone like... *opens IMDb* Shawn Parikh or Paull Walia.