Although Math Formula cannot interact with strings, it’s not totally the case that String Manipulation cannot handle numeric formats, in spite of its name. 
It’s often underutilised as a node because it isn’t always obvious what it can achieve.
It is correct to say that it cannot perform string replacement directly on a Double, but it can use it in string transformations, if it first converts the Double to a string.
So the following would return the Double converted to an Integer using string replacement, as an equivalent of @lelloba’s Column Expression:
toInt(replace(string($column1$),".",""))
and it can also do the equivalent of @tqAkshay95 's Math Formula approach:
toInt($column1$ * 1000)
see here for more discussion on String Manipulation “hidden features”