A decrease in the assessment doesn't phase in like an increase would, but because of the Homestead credit, the amount you pay may still rise even though the assessment goes down.
Here's an easy way to get a sense of whether your taxes will still go up or not. Look for a 'CHCR' listing in the left hand column of last year's tax bill. If there's no amount listed for it, or there's no listing for 'CHCR' at all, then your taxes should go down (if you're in St. Mary's County, by roughly $500). If the negative amount listed is less than about $425, then your taxes should go down. If the negative amount is much more than $425, then your taxes should go up. As a general proposition, the larger that credit, the more your taxes will go up this year, and the smaller that credit, the less your taxes will go up this year (or the more they'll go down).
Don't take that for gospel, but it should paint a roughly accurate picture (and it's based on your assesment going down $50,000 - it won't be accurate for others whose assessment went down by a different amount).
