I hate things like that in writing - in your shoes, my tendency would have been to allow the Plat to expire (Feb 28 2006) and a couple of weeks or more after, CALL and ask to be signed up for a Plat challenge (or if more advantageous, perhaps after June 15, given that would confer Plat for the remainder of 2006, all 2007 and through Feb 28 2008.)

Altertnatively, you can buy your way through Feb 28, 2007, but will not be able to repeat a buy-in in 2007. Of course, it appears you will re-earn Platinum with no problems this year, and that "honestly earned" status will also persist through February 2008. A call to challenge so you can hold "challenge" status until you earn BIS-granted status might snag you a better position for upgrades, some more bonus miles, etc.

I'd never have written, frankly.