It depends what you mean by "automatically"

If I have a number of cells  on a sheet which contain numbers  formatted as text
what I do is:

Type 1 in a blank cell
select the cell and copy it (ctrl-C)
select the cells to be changed
edit/paste special/multiply
OK

This seems to work.

If you want to get a macro to do it you could use a similar method. It would
depend on
- how you were going to identify the cells to be changed
- how the macro is to be triggered

The decision on the method to be used would be based on the frequency with
which you had to do this.