There's no rule saying you can't go back and rework "finished" models.
I completed my Riptide late last year, and created a custom head using extra bits from the kit. While I was please with the way the face turned out, soon after I completed the model, something about the head started to bother me.
I realized that my custom build included two antennae, when the rest of my entire army only had one. It's consistent throughout the force - a single antennae on the right side of the head. Fire warriors. Pathfinders. Stealth squads. Crisis Suits. Broadsides.
I carefully removed the head from the model, sliced off the both antennae, actually, and replaced them with a single unique one from the kit. It was a delicate procedure to ensure that I didn't damage any of the areas that didn't need to be changed, but after a quick snip, some precision x-acto shaving and a dab of paint to retouch the surface - it was complete.
Definitely looks better.
What do you think? Have you gone back to "fix" a completed model you weren't happy with? Share in the comments below.