You would have to go to your database and export your field as a CSV file. Then open file with excel.

In Drupal delete the field and create a new decimal field. Go to a node and enter a value for your decimal field. 

Go back to your database and find the decimal field, export as CSV. Then open on excel.

Use the values from your 1st excel to copy the node id's and the field value and pase them onto your 2nd excel and save.

Import 2nd excel to your decimal field and check-mark replace table data with file. 

Clear/flush to cache to see changes.