Hmm. I don't think it's a VFX (animation) error. Take a look at the rear of the Reliant
and compare to the impulse engine on top version in DS9
The DS9 version's torpedo pod in the rear is divided completely in half by a rectangular hull piece which physically looks like an impulse engine. The Reliant's pod has a cylinder dome that extends back but only covers up the top rear and doesn't divide it.
The DS9 version's impulse engine on the body has the "red exhausts" only half as tall as the ones on the Reliant so they have a different impulse engine setup.
You could say the modeler goofed or changed the design but I don't think it was a goof on the part of the animator.
Anyways, the DS9 ship does have different physical features regardless of the animation fx to think of the rear of the pod as something that appears like an impulse engine, IMHO.