There's not really a definitive guide for how "modified" a mod needs to be to qualify as a demo. Even if your mod just has one or two graphical changes, map script changes, or anything simple like that, then it's a demo that should be allowed in the showcase section.
Instead of viewing it from a standpoint of whether or not a demo that short is allowed to be posted, try viewing it from a standpoint of whether or not the demo has enough content for community members to give meaningful feedback.
The only thing that should stop you from posting a demo is whether or not you think it's ready to get tested and critiqued.
