I am currently an Exempt worker in California. I am being sent to perform hourly work at a store during a remodel for 2 months. During that time I will not supervise or be performing any exempt duties. Is it lawful for them to use me to save overtime pay that they would have to pay hourlys.

What type of work are you required to perform? If your work is more than 50% manual and you are not making any sales, etc., then you are probably non-exempt and, therefore, you are eligible for overtime.

You can read about it at: http://www.dol.gov/elaws/esa/flsa/screen75.asp California is the same except it requires a higher level of pay.

My typical job duties are that of an Operations Manager with a minimal amount of hourly work being performed. But for 6 weeks, 2 months ago and now for 2 more months I am being required to perform hourly duties 100% of the time.

Then you would be considered as non-exempt. There are a few exceptions for those of commission. However, the person must still earn a rate equal to minimum wage and overtime.