In W-OTS, is it trivial to forge smaller N values?
W-OTS has a checksum that ensures that kind of attack doesn't work. 

In essence, the checksum is the sum of the value of every other component subtracted, so that if you hash one component one more time, you need to find a pre-image of a component of the checksum. 

Basically, it looks a bit like this: 

[ - | - | - | down | checksum up ]


Related: https://crypto.stackexchange.com/a/63903/36960