How to change scrypt parameters?

[ir.hn]

This is a general crypto question it isn't restricted to any particular crypto.

How do you change modern scrypt parameters N and P and the like?  On Tenebrix code these are listed at the bottom of scrypt.c but in litecoin and even the reference scrypt code these parameter settings are missing.  Is it even possible to change the parameters anymore and if so how?

Will donate a bit of bitcoin to anyone who can help out.