Runtime mesh decimation

There are many things not clear in your question. First of all what do you mean by "decimate"? A $$anonymous$$esh simply consists of vertices and triangle definitions between those. If you aim for lowering the triangle / vertex count for arbitrary meshes that's not possible. This is generally called "retopo" but usually requires user steps in order to produce any useable output.

Second as mentioned already, what kind of meshes do we talk about? Even if you find some automatic way to reduce the poly count it won't work for an arbitrary mesh. If you remove / reduce certain parts of a mesh it will no longer look right. A good example is the Utah $$anonymous$$pot (i made this one). The Utah $$anonymous$$pot however is 100% procedurally generated from nurbs surfaces (you can see the control points when you enable "control"). If you set the U and V resolution to the $$anonymous$$imum the lid handle would basically disappear. Also the spout as well as the handle would be just a flat surface.

Finally you haven't mentioned what the overall goal is. What does the framerate has to do with this? Reducing the triangle count doesn't really increase performance. The number of drawcalls / setpass calls usually has a much greater impact. Since Unity is limited to 16 bit index buffers a mesh is limited to 65k vertices.

The question is way too unspecific. You should edit / improve your question to make it more clear what you actually want to do. UnityAnswers is for concrete technical questions about the Unity engine and development with the Unity engine. Yes you can manipulate a mesh at runtime in Unity. The rest of your question seems to be related to an algorithm which you didn't specify.

So if someone comes up with an answer for you then you are committing Plagiarism and can get you kicked off your course and Arrested! Sorry to tell you this.

In my humble opinion what you are trying to achieve defeats the purpose : $$anonymous$$esh manipulation is pretty intensive given the work required. To perform this as a cycle-saver surely would not work as a result.

You will need to perform Decimation in advance (Blender makes this easy) and save the models ready for use. Its then a simple case of enabling and disabling GameObjects to swap out. I know this is not what you're after but this is how its done in AAA. Good luck in your endeavours.