Hi Sweta,
1. You need to create new schema by copying the old one.
2. While creating new schema, maintain the timing in future say for eg., 30 min
3. If you want to add new fields / delete new fields you must do it with in the time frame of 30Mins
4. This means, based on the quantum of work, you have to mention the time in future
5. The moment 30 mins are over, automatically the new schema will be activated and the old one gets de-activated.
Rgds
Hari