My status changed from non-resident to resident last summer, 2009 however, I received 1042s form for treaty benefit.

Should I use 1040NR for tax return this year?

Also, last year I filed 1040NREZ for tax return and I wonder it was a correct filling since I doubt about my status change.

I stayed in US for following days

2009-326days
2008-366days
2007-284days
2006-365days
2005-139days
1998-194days

Why did my status change in the middle of the last year? My tax filling was correct last year?

Your visa in 1998 and in 2005 to present?