Of course not. That's just a fancy way of saying "cheating." I mean, honestly:..
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"Oh, no, honey, I wasn't *cheating* on you when I slept on Raoul. I had just taken our relationship status from 'exclusive' to 'non-exclusive.' Didn't you get my email about that?"..
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Semanticize it all you will, unless you have an open relationship -- and both parties in the relationship are apprised of that fact beforehand -- that's plain ol' infidelity...

Nope, not acceptable.....
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As for when she should tell him, it depends on the girls level of honesty, integrity and decency but I'd say, generally, somewhere between :..
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a) she decides that's, in fact, what she wants to do, and..
b) she does anything with anyone else that she would normally reserve for her "exclusive" bf (kiss, pet, wear his letter, meet him by his locker every morning lol) ..
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I've going to presume ur young and ur asking because u'r at a crossroad ... so what I would tell you is this, which actually will likely help you at almost any crossroad, in just about any relationship, you value or are trying to cultivate .....
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1) When in doubt, put yourself in the other person's shoes and ask what you would want done to you...
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2) Conduct yourself in a way that allows you to walk with your head high and look people in the eye including, and most importantly,..those eyes you see lookin' back at you in the mirror...
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Good Luck..
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Angie..

No, that sounds like cheating if you had agreed to be exclusive in the first place. You tell him when things have changed WHEN they've changed. You tell him that you don't want an exclusive relationship anymore or that you wish to break up and date others. Sounds like a cowardly way of weaseling out of the relationship instead of being honest and upfront about the status of the relationship...

I believe in being honest -- and for everyone to know what is going on. If you don't want to be with someone anymore -- or you want to see other people you should tell them BEFORE you do anything with someone else...

If you were exclusive, it's dishonest if you don't tell him that the relationship is changing to non-exclusive. I think as soon as you decide to start seeing someone else, you owe it to the previously-exclusive partner to tell him...