How to set a number number value as text in all cells ?
 

I have exported data in text format. Cells are set as text. When numbers are
input into the spreadsheet, the format changes. When the cells are
reformatted to text, a check with ISTEXT is false until the cell is edited
and the format accepted. I want to set all cells after they have been
changed/values input back to text and not cell by cell. How ?