Lost residue modifications

Hello,

I'm trying to modify the residue vectors in Vorbis audio files at
encoding time. For testing, I'm just adding +1 to each residue vector's
element, so e.g., if I have an original residue vector of [0,1,2,3] I
modify it during the encoding to be [1,2,3,4], and at the decoder's side
I should receive the [1,2,3,4] vector. The problem is that, after adding
the +1, I still receive the vector [0,1,2,3].

I'm making this +1 to each element in a simple for loop for each
channel, immediatly before the line number 700 of mapping0.c (just
before calling _vp_couple):

for(count=0; count<psy_look->n; count++) {
  vb->pcm[i][count] += 1;
}

Obviously, there is some place after in the code where my changes are
being overwritten or somehow discarded, but I am being unable to find it
(I thought it would be where a comment line says "couple_bundle is
destructively overwritten by the class function if some but not all of
the channels are marked as silence; build a fresh copy", but there is no
silence in any channel).

Could someone give me a hint on what may be happening, where my changes
are being lost or where should I modify the residual vectors... anything.

Thank you.

PS: If it is important, I'm using quality level 6+ to force lossless
coupling (AFAIK, 6+ quality levels are the ones with lossless coupling,
right?).