When you change the format of cell/range, the change will be actual only after you edit the cell(s). Until then the old format is applied really.
So when you have e.g. a column with numbers or dates formatted as text, and you format the column to General or Date, after formatting the column you have to edit all entries in column. The fast way is enter 0 or 1 into some free cell elsewhere, copy the cell, and then PasteSpecial>Add or PasteSpecial>Multiply the formatted column.
When you want to format numeric column to text, a foolproof way is after formatting to enter the formula
Code:
= "" & CellReference
into free column, copy it down, copy the range with formulas and overwrite values in formatted column with PasteSpecial>Values.
With one or couple of cells only, simply double-click on cell, or activate it and press F2, and then press Enter.