Ok the first thing to remember is that they both perform different functions

Lets start with ISNULL statement

WHERE ISNULL(Col1,'') = 'NOCHANGE'

The reason you use is null is so that you don't get thie

WHERE NULL = 'NOCHANGE'

instead you will get:

WHERE '' = 'NOCHANGE'

So in your example you possible clause are as follows:

WHERE '' = 'NOCHANGE' (this is FALSE)

WHERE 'anything' = 'NOCHANGE' (this is FALSE)

WHERE 'NOCHANGE' = 'NOCHANGE' (this is TRUE)

Now using the NOT operator basicallt says do the opposite

so NOT TRUE = FALSE

and NOT FALSE = TRUE

Does that make sense to you?