Bearing in mind the Gezeirah Shavah "Tzara'as Mam'eres" "Tzara'as Mam'eres" (from Nig'ei Begadim) which teaches us that the room is Tamei even if the new Nega has not spread, why does the Torah write "ve'Hinei Pasah ha'Nega"?
Rashi: This Pasuk belongs after Pasuk 45, 1 and, via a Gezeirah Shavah "u'Va ha'Kohen" "ve'Shav ha'Kohen" (Pasuk 39) teaches us that, if the Nega returns after the second set of seven days, the Din follows the same pattern as after the first seven days (in Pasuk 40), and the owner is obligated to replace the stones and practice a third seven-day quarantine. 2
Ramban #1 (on Pasuk 43, citing Toras Kohanim): "u'Fasah" means (not that it spread, but) that it erupts 3 (a second time), 4 to emphasize that it is not a continuation of the old plague, but a new one. 5
Ramban #2 (citing Targum Onkeos): "Pasah" and "Parach" are one and the same, only the former is speaking where the existing Nega expands, and the latter, where it erupts. 6
See Ramban's objection to this explanation (DH 've'Lo Hutzrach).
Like "u'Parach" in the previous Pasuk. See Ramban, DH 've'Yitachen'.
And the Pasuk is saying that, whether the new plague erupts on the seventh day of the first quarantine, immediately after replacing the stones and the mortar or after the termination of the second quarantine, it is Tamei (Ramban).
See Ramban DH 'Veyitachen'.