|January 24th, 2015
From their perspective this is an improvement, both ethically (cheese production involves much less suffering than meat production) and selfishly (mmm, cheese). From my perspective this is ethically neutral (I don't think eating meat or cheese is wrong) and kind of complicated selfishly: I'm giving up some meat, but only occasionally and only the meat I'm least enthused to eat. On the other hand, I get some enjoyment from their eating cheese (they're a close friend) and from making trades like this (gains from trade are weird to talk about if you just like trading).
This trade is a kind of ethics offset, a partial version of paying someone else to go vegetarian, which does mean there are counterfactual trust issues. The biggest question is, how do we know what meat I would have eaten otherwise? My approach here has been to only occasionally avoid meat for trading purposes, so that I keep up a sense of how much I typically eat. Then when I'm about to serve myself some, I either take half as much or if that's not an option flip a coin and either take it or not. If I look down at my plate at lunch and realize I've happened to choose an entirely vegetarian meal, without making any intentional choices to pass up meat for trading, that doesn't count for trading (and it should not).
(Some people have an internal sense of how much of different foods they've been eating lately and find themselves equilibrating over the course of a few days. For me, however, any balancing happens over a much shorter timeframe, like a few hours, so I'm not worried about this.)