If you use vl code you can get "fitpoints" eg FitPoints = (-2.0 142.0 0.0 117.0 94.0 0.0 220.0 153.0 0.0 330.0 94.0 0.0 388.0 155.0 0.0) this is the XYZ of the points making the spline, you could change one of the points. The pts are XYZ.
(setq pts '(-2.0 142.0 0.0 200.0 117.0 220.0 153.0 0.0 330.0 94.0 0.0 388.0 155.0 0.0))
(vlax-put obj 'Fitpoints pts)