Requires Left4Dhooks, but even if you can't use it for whatever reason, find snippets for VScript.
FormatEx(TempFormat, sizeof(TempFormat), "GetPlayerFromUserID(%i).SetModel(GetPlayerFromUserID(%i).GetModelName())", GetClientUserId(victim), GetClientUserId(victim));
victim is the player in question.
Getting up means getting up from whatever animation player is in, with the sole exception of tank rock hit.
Getting up from hunter
Getting up from charger post-charge.
Getting up from charger after impact
Getting up from Tank after Punch.
Getting up from Bitch Slap, as exists in some plugins.
Please verify you're doing it on a non-incapped player.
This also frees a Special Infected from stagger, so if Left4Dhooks didn't have unpin method, you could stagger into this function.