On 2007 I filed my 2006 tax return as 1040nr ez because I was under f1-opt visa. However, I just learned that I could have filed as resident alien (1040) because I was in the country for the entire 2006. Is this correct? If it is, can I file an amendment to my 2006 tax return, while filing my 2008 return, and then claim the lifetime education credit?
Thanks.

No, that is NOT true.

International students in country under the F-1 visa are EXEMPT from the Substantial Presence Test (SPT) for up to five years while under the F-1 visa.

Assuming that you arrived in the U.S. on or after 1 January 2002, you were still under that exemption in 2006, so you filed correctly when you submited Form 1040NR-EZ for 2006.