Hi Praveen,
Please check your status profile customizations.
You are tying to change the status from Complete to Open. In your status profile check the lowest status number & higest status numbers.
For example for open (10) you have status Lowest as 10 & higest as 40 in this case you can set the status from 10 to 40.
For example your completed status have 40, in lowest you should have 10 & higest 40. In this case you can set the status from 40 to 10.
And also check authorization code for the completed status.
Thanks & Regards,
Ramesh.