Can not change series in sales invoice

I tried to change the series in Sales invoice and got me above error.

All invoices are in Draft status.

Please help me to resolve this.

Umm, you cannot change the series after saving a document and expect their IDs will be changed. Once saved, the database has an entry with that series and number appended. You will have to redo all these invoices.

I can suggest a solution which should work (try it on a spare testing server). Export all the sales invoice in excel. Change the series as you want. Then re-import the excel file.