Hi,
I was on F1 from 1998 to 2004, F1 OPT from July 2004 - July 2005, and H1 since July 2005.
I'm working at the same job since my F1 OPT (so I have one W-2 form).

In 2005, I was in the US with H1-B for less than 183 days, but I have been hear long enough with F1 visa, so I don't know if
1. I have to fill out 1040NR or 1040 (Last year, I used 1040).
2. I can apply a tax treaty for H1 holders or not (My home country (South Korea) has a tax treaty with the US for F1 students for 5 years (which I used as a student), and another tax treaty for H1-B holders (for 2 years)).

Can anyone help me?
Thanks!

If you want to file Form 1040NR for 2005, you need to amend last year's return to convert to 1040NR for 2004. Otherwise, you must file Form 1040.

I do not know about the tax treaty provisions. Suggest you access www.thetaxguy.com. He specializes in tax returns for visiting aliens like yourself.