I am looking for a formula that will leave the prior response from the
formula when a key cell changes... for example.

I have 2 sheets in my excel document.

I am pasting the current months data on sheet 1. In cell A1 of Sheet1 I
have the current month, "May". When this matches the month in a given row in
column A on Sheet 2 then I want the VLOOKUP to look up a value. (I know how
to do up to this point) When June comes I will be replacing Sheet 1's May
data with June's data. I then don't want the May data to be lost on Sheet 2
even if all of the May data is deleted from Sheet 1 and replaced with June
data. I know you can paste values but I would like the formula to stay even
if A1 on Sheet 1 does not match the the current month in Column A of Sheet 2.
When they don't match I want it to leave the value that was in it before
instead of changing it. Is this possible?

Sheet 1
Contains a bunch of data which will be used to fill out Sheet 2.
A1=Current Month "May"

Sheet 2
Contains a table that will need to be filled out.
A1= JANUARY B1=Response
A2= FEBRUARY B2=Response
A3=MARCH B3=Response
A4=APRIL B4=Response
A5=MAY B5=Response
A6=JUNE B6=Response (This would be zero until June then it will
update with JUNE's data. In July I want it to stay the same value and not
change)