formulas changing unexpectedly

I have the following: Excel 2010 - In column A it is a linking formula to column B but col A is formatted to show the month of the date in column B
The problem I am experiencing is if I move the data (let's say with a drag/drop, or even a cut/paste)from B2 to another cell (say B4), the formula in column A changes cell reference.
Where I was going with this is I wanted to use a formula starting in A2 like this =IF(ISBLANK(B2)," ",B2) so if cell B2 was not filled in, the cell in column A would remain blank, otherwise put in the month of the date

1 Col A Col B
2 =B2 2/3/17
3 =B3
4 =B4

Can anyone help? I may be missing something simple.