Meh, depends on the % match and up to what % of pay.
Changing it on you is less of a sting if they keep those values the same, and is probably cheaper/easier on the employer to take an annual deposit number and your 1099 and do the math at once instead of continuously.
Just FYI, my company's match is shit ( (1) 5% to 15% depending on "generosity" that year, (2) paid at end of year, (3) up to 7% of your pay, (4) with a 5 year vesting schedule type of shit), but I knew that going in and worked there anyway.
I wouldn't leave over it, but you bet your ass that I'm leaving after it hits the account without giving 2-weeks. Doing crap like that incentives disloyalty in the long run.