I'm currently having H1B visa and my wife is on H4 visa since 3 months.She is planning to change her visa status from H4 to F1.Will I have the same tax exemption as I have now though she changes from H4 to F1?

A spouse is never a dependent.
On an F1 visa, the person is exempt from counting days for the substantial presence test for 5 years, so she would be a non-resident alien for that time. Non-resident aliens cannot file a joint return.
However, if you yourself are a resident alien, you can both choose to treat her as a resident alien as well and you would be able to file a joint return.
Have a look here:
http://www.irs.gov/businesses/small/international/article/0, id=96370,00.html
As a resident alien she will be subject to social security and medicare on wages earned.

I'm currently having H1B visa and my wife is on H4 visa since 3 months.She is planning to change her visa status from H4 to F1.Will I have the same tax exemption as I have now though she changes from H4 to F1?

You can file joint return as residents.