Not so much A but B

When we deal in connotation, we are dealing with synonyms that share pretty much the same basic definitions. What changes, as you move from one synonym to another, is not so much the meaning but the effect.
[Source: Reading for Results Ninth Edition by Laraine Flemming]
I'd like to know if I can replace "but" with "as" without the change of the meaning; I think I can.
Thank you in advance for your help.