Why is Find/Replace saying "There's a problem with this formula"?

Hello,

I am boggled by this and it's hindering me from replacing something within a formula across a worksheet that should be fairly simple of a process to complete.

I have this formula in a cell that refers to a table within another worksheet:  =sumifs(Jan5Wk1[Amount],Jan5Wk1[Product],"A*") 

I am trying to do a Find/Replace to change "Jan5" to "Jan6". 

When I click "Replace" within the cell I am returned with the "There's a problem with this formula. Not trying to type a formula?...etc" error.

First, I am trying to type a formula.  

Second, if I manually type the formula =sumifs(Jan6wk1[Amount],Jan6wk1[Product],"A*") Excel accepts it.  

I have tried entering the entire formula in for both the find and replace and that returns the same error. 

I don't want to have to go to each cell and type this formula when the formula is already intact and the only part of the formula I need to change is the "Jan5 to Jan6".  It will take me forever to complete this project if I have to do this plus it leaves room for error in inputting this formula in each cell.  I have 5 worksheets to change like this in a total of 1200 cells.

What is wrong with Excel that it is returning this error?  What am I doing wrong that would cause this?

Is there another way to find and replace data in a cell other than the Find/Replace feature?

I appreciate any help you can lend with this situation.