Ctrl H or Finding / Replacing

Cannot do a simple Find and Replace in MS Excel 07.

Do a lot of week to week charts and the reference letter changes week after
week.
When I go to update and replace last week's numbers with this weeks numbers
- I get the Does not recognize...

Same format only the letter is changing - very careful about just taking out
the letter only. Have tried cutting the cell contents and placing in the Find
and then doing the same in the Replace and changing the letter in the box.

Nothing works