Hi,
I'm a french research scholar working in an university in Chicago, IL.
I arrived in the US in Feb 2006 with a 3 years J-1 visa and based on a treaty between France and the US, I was exempted of paying income taxes during 2 years, so 2006 and 2007.
I changed my immigration status in April 2008 to a H1B visa. At that time I wasn't anymore exempted of paying taxes but I'm concerned about how this change in my status could affect the 2 years exemption I had before that.
Thanks for you help,

On J1 you are exempt for two years from the date of arrival in the U.S. So it is not 2006 and 2007. But for two years from the date of arrival in 2006.
The treaty does not say that if you stay for more than two years, as previous benefit will be lost. So it will not affect the 2 years exemption.

So to be exact it is from feb 2006 to feb 2008 then.
I know for some countries, if they stay more than 2 years they lose their entire exemption from the beginning. But treaty with France doesn't seem to point this out, as in your answer.
Thanks much!