Changing field from int to char in table when database is on

It's not possible to directly change a field in that manner -

The process to make that change would be:

1) Rename field

2) Add new field with the correct data type

3) Copy the old field data to the new field, type corrected

4) Add indexes which use the new field instead of the old field

5) Delete indexes that use the old field

6) Delete the old field.

7) recompile application