So I am building up my Seraphon army, and I would like to include the Firelance Starhost. It calls for three units of Knights and a Scar-Vet. Knights are purchased in units of 8, and two would equal 16. Therefore, I would like to convert one to a Scar-Vet, to have the exact minimum for the Starhost. How do I do so? (I couldn't find anything to answer this exact question). I'm new to AoS as well as this forum and I've only recently finished painting my first models. I would greatly appreciate help.
Give him the fanciest head sculpt you've got, and the war pick from the knight kit and the rest is up to you. As long as he stands out from your other knights whatever you do should be alright with most opponents.
I gave him the big shield from the Carnosaur kit (in case you built a Oldblood on Carnosaur you should have it) and gave him the included war pick, which I "pimped" a bit using a few spare feathers from elsewhere that I glued on there and painted in some easily visible color. Here it is: http://lustria-online.com/threads/aginors-painting-blog.19114/#post-181100
Whatever makes it stand out really, but yeah typically it would be one of the scar-vet heads from the Carnosaur kit and maybe the big shield, or the chest plate.
You could also bulk out the base a little. Stick the cold one on a wood chip or some cork then paint it like a rock. Your scar vet will look bigger and more imposing even though it's the same size.