Changing Job Before B-School application

Generally speaking that will hurt your candidacy. The adcom is likely to wonder if you either 1) weren't honest with your new hiring manager -- very few people will hire you knowing you plan to leave in approximately a year OR 2) aren't that serious about b-school (since you just took the new job). The exception is if you take a job at a place that you plan to name as your ST target company following b-school. Then you can make an argument that you can spend this next year gaining exposure that you can leverage when you return (armed with the MBA) ready to make bigger impact. The adcom will also believe that a company is willing to take you on for a year if you ARE planning to come back & make a bigger impact following b-school.

In addition to those big reasons, another reason to stay put at your current job is that it usually gives you more capacity to really do your best on your applications vs. having to prove yourself at a new company. But honestly the most important factor is just the risk to your candidacy.