May the first be with you

f=
s=>s.replace(/force|first/gi,s=>s.replace(/./g,c=>s[s.search(c)^1]||c,s="oicsetOICSET"))

<textarea oninput=o.textContent=f(this.value)></textarea><pre id=o>

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

Retina, 33 bytes

iT`\OC\E\ocetsiTSI`Ro`first|force

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Edit: Saved 5 bytes thanks to @MartinEnder for pointing out what Ro does.

Requires ⎕IO←0 which is default on many systems. Can be four characters shorter using the Unicode symbol ⍠ instead of ⎕OPT .

(t←'force' 'first')⎕R{(m∊⎕A)c¨t⊃⍨~t⍳(c←819⌶)⊂m←⍵.Match}⎕OPT 1

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