You can change it any time you want, or as often as you like threw the year.
When I worked construction and did a lot of shutdowns at sites a lot of the guys would put 8 I think it was on their W2s so all that overtime didn't put you into the 50 % tax bracket and then when the overtime was over go back to just 0 or 1, most of those only lasted a month or 2 tops and by the end of the year they said it evened out so they didn't owe anything I tried it once for a month and then went back to 1 and come the end of the year I still got a refund. But being married I also had at least 3 legitimate deductions every year as well.
Steve
[This message has been edited by 84fiero123 (edited 06-05-2014).]