The day you reach the threshold for an Elite segment is the day you receive that status.
If you reach 75,000 miles on May 20, 1999, you will be considered Platinum Elite from May 20, 1999 through February 28, 2001 -- the rest of this year, all of next year, and Jan/Feb of the following year.
When you reach Platinum, you'll get 125% mileage bonus for miles earned while Platinum. You'll earn more miles, so there is a benefit to continuing on CO.
When you hit 25,000 miles this year, you'll be Silver. You are on your way up.