Modify value of a cell using the old value as part of the new one?
Solution 1:
Presumably, this is something you only want to do once, precisely because it makes no sense to have a circular loop in your formulas causing the values to be divided by 1000 every time you recalc the sheet (even supposing Excel would let you do that.)
So here's what I would do: Create a new row and copy a formula dividing the original value above by 1000 into each cell. Copy the new row and then "Paste special ... values" into the original row. Then delete the new row.