Unable to update period names in GL accounting calendar, that were incorrectly named

Our accounting year 2016 is from 1 July-2015 to 30 Jun-2016

The year is divided into 12 periods (months)

 

Problem:

Our period names for accounting year 2016 were incorrectly defined

Example: the first period that spans 1-Jul-15 to 31-Jul-15 was named Jul-16 when it should've been named Jul-15

 

When we try to change the period names we get the following error:

"APP-SQLGL-08044: You can only modify periods that have never been used"

 

Tomorrow is 1st July-15 and the beginning of accounting year 2016. Our client is not wiling to use the periods with incorrect names. Five group companies are using this calendar and all their entries will be on hold from tomorrow. We need a solution urgently. Kindly advise on possible options.