I don't necessarily disagree with anything that's been said here, but I have had a different experience - I think in certain special circumstances you can switch committments without damaging your career.
When I was still in grad school, I was offered a non-union job as an ASM for a small summer theatre festival. I said yes to it with an eye towards working at this festival in the future. Shortly thereafter, I was offered conflicting job as an ASM (not one that I had applied for or thought that I could get) that would turn me Equity and pay me more than three times as much for a shorter contract. There was also at least two months before the first contract was to start.
I called up the first job and explained the situation to them; I think if I had been anything other than totally apologetic it would have gone badly; I also didn't call them up with a fait accompli, I told them that I had this other offer and I wanted to take it but first I needed to make sure they understood.
Later that year, the first theatre called me up and offered me the Union SM position for the following season. I said yes on the spot, and lived up to that commitment. I was flattered and honored that they didn't hold my previous decision against me. After I finished that season with them, they said I was the best stage manager they had ever worked with and we agreed on a contract for the following summer as well!
So here are the circumstances that made it okay:
-I didn't have a signed contract;
-I hadn't been looking for other work behind their back;
-The conflicting job was worlds better for me than the first job;
-I gave more than two months notice, before any stage management work had been done on the season
-I had not yet met anyone (including the stage manager I would have been assisting)
In combination with that, it was an ASM position, and relatively easy to replace. The personality of this particular producer came into play as well.
Since then, I've been a lot more careful about committing to jobs. Usually I say yes only after I know I've at least explored other options. For very early offers, I usually ask for some time to consider before saying yes/no, and producers have been understanding about that.
I'm in favor of the MRE clause in most small Equity contracts, and I appreciate the hard negotiating that got it in there, but I do think it's playing with fire. A stage manager (as opposed to an actor) who made use of it would have to have weighed the situation very carefully, I think.