"among the left" is a really wide net to throw and I don't think it really does any favors to your statement to be so overly general.

I think it's pretty much the mirror image of the Benghazi investigations. Some investigating needed to happen, but politics got involved and turned the whole thing into a circus. This isn't particularly surprising since the US's two party system usually leads to extremely incompatible "truths" now that social media bubbles exist.

I'm pretty sure the left dislikes James Comey quite a lot since he did a last minute "give Trump's Hillary emails conspiracy a false sense of legitimacy right before the election" thing

"The left" is multiply defined. If you mean Democrats generally, the party has become the number one home of middle class people with college degrees. They are naturally going to be more comfortable with the FBI than, say, old school left wing labor activists.

It seems less likely that opinions are changing, and more likely that people with certain opinions are moving.

Both of these guys have been involved in really shady stuff in America's past, they are career company men and never represented or mounted any real threat to Trump. Comey may even be a big part of how Trump even became POTUS in the first place.