there's a way you can do it, but it's a few steps. in your srb2/srb2k models folder, find the player model you want to replace and rename them to back them up. then, place the resources of the model you want to use in there.
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to make this even easier, just make a sub folder to place the model you're not using in, and swap them out at your leisure.
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afaik there's no way to swap them out in-game without some third party lua i'm not aware exists.