\version "2.12.2"

\paper {
  indent = #0
  ragged-right = ##t
}

expandSlur =
  #(define-music-function (parser location input) (number?)
    #{
      \once \override  Slur #'control-points = #(more-curve $input)
    #}
)

#(define ((more-curve internal-offset) grob)
   (let* ((coords (ly:slur::calc-control-points grob)) ; get control-points

          ;; break list into component pairs
          (first-pair (car coords))
          (second-pair (cadr coords))
          (third-pair (caddr coords))
          (fourth-pair (cadddr coords)))

     ;; adjust y-coordinates of middle points
     (set-cdr! second-pair (+ internal-offset (cdr second-pair)))
     (set-cdr! third-pair (+ internal-offset (cdr third-pair)))

     ;; reassemble control-point list
     (list first-pair second-pair third-pair fourth-pair)))

top = \relative c'' {
\clef treble
\time 3/4
\key c \major
%default slur
c2 ( d4 e8 d c b a g )
\expandSlur #2
c2 ( d4 e8 d c b a g )
\expandSlur #5
c2 ( d4 e8 d c b a g )
}

\score {
  \new Staff <<
\new Voice { \top }
  >>
}