I bought the "Start Collecting Box" containing 10 Saurus Warriors and they did not perform very well or at least worse than when used as Saurus Guard proxies instead. So the next step would be to convert my Saurus Warriors into actual Saurus Guard models, but is that even possible? As far as I can see from photos both models have nearly the same size and all that is missing is a conversion of the weapon, a skull-hat as well as some bones and other decorations on the shields. Is that correct or do they also have body armor?
If you're not fielding any other warriors, 95% of people you play won't mind if you play them as guard. If you wanted to upgrade them, you could use the totems feathers, dangly skull bits, different weapons attached to their hips etc. to bulk them out a bit and make them look fancier.
10 warriors will always be lack lustre, they work best in larger units backed up by a Sunblood, Guards are better in smaller units but also need buffing by a hero or Eternity warden. If you want to convert look for a bits shop for Guard heads and weapons.
You can also buy a box of guard, there are extra heads and bits in there to make the conversions pretty easy. Take the tip off of the spears and put the clubs on top, looks like the guards halberds, the conversions should go fairly easily.
There are some extra warrior models in the SC box. Lets see what a little bit of Greenstuff can do for me. However: Do guards wear some kind of body armor? For some reason all photos I found use nearly the same angle with a good look on the shield.
The shield is held closer to the body on the guard. And they have skull helmets. That's the only real difference.
Saurus guard bodies are slightly thicker, I learned this as I had the same idea as you. If you take the leftover champion shoulder pads, they make good bits of chest armor for the warriors which makes them look bulkier.
The Guard also wield Halberds, which you can easily convert by giving the Saurus spears, cutting off the tips and replacing them with the ends of the clubs.