Um, I'm just going to give my own opinion on this...

I think a canon character can be changed [i]if[/i] it's justified. For example, being summoned in an alternate class, or, say, if you're using Jeanne d'Arc giving her an NP that doesn't kill her so she's actually viable as a Servant.

The thing with Gawain is that he's deliberately a broken mess in /extra and that's why I feel like using him is kind of sketchy without serious alterations. Since I talk to Rain a lot I think that's probably part of the reasoning.