May the first be with you
JavaScript (ES6), 93 88 bytes



f=
s=&gt;s.replace(/force|first/gi,s=&gt;s.replace(/./g,c=&gt;s[s.search(c)^1]||c,s="oicsetOICSET"))
&lt;textarea oninput=o.textContent=f(this.value)&gt;&lt;/textarea&gt;&lt;pre id=o&gt;




Edit: Saved 5 bytes by optimising the unchanged letter case.