Value 1 is what you want to do when the condition is true. In our case, we want to disaggregate the data. Calculating the difference between a value and a previous value of a series can be achieved with the formula:

momentum(series, length)

where length is the observation window between the value and the previous value. Here, you want to subtract the value of the previous month, so the length is 1.

If(month()>2, momentum(cntrad1290, 1), value 2)