This method use export and import function.
NT exports contract into "symb Expiration" filename, for example: ES 12-09.txt, which is export data file for December 2009 emini contract.
The method is simple. You rename the name of the exported file into the name of the contract you wanted to import to.
For example, you want to import ES 09-09 to ES 12-09. So you export ES 09-09 with your date range. Then you rename ES 09-09 to ES 12-09 and select import.
The data from the file will be imported to ES 12-09 unaltered.
Hope it help someone.