Mishnah Negaim 7:1: These Beharot [type of diseased patch that creates impurity] are pure: Ones that existed in him before the Torah was given, on a non-Jew who [then] converted, on a child or a newborn, on a fold [of skin] that was uncovered, on the head or on the beard, on a boil or burn or blister that is unhealed. [If] the head or beard returned and became bald, [or if] the boil or burn or blister [returned] and grew a scab, they are pure. [If] the head or the beard have not yet grown hair [in them and then] they grew hair and [then] balded, [or if] the boil or burn or blister have not yet grown a scab and [then] they grew a scab and [then] healed, Rabbi Eliezer ben Yaakov declares impure because [both] initially and finally they are impure. But the Sages declare pure. 

אֵלּוּ בֶהָרוֹת טְהוֹרוֹת. שֶׁהָיוּ בוֹ קֹדֶם לְמַתַּן תּוֹרָה, בְּנָכְרִי וְנִתְגַּיֵּר, בְּקָטָן וְנוֹלַד, בְּקֶמֶט וְנִגְלָה, בָּרֹאשׁ וּבַזָּקָן, בַּשְּׁחִין וּבַמִּכְוָה וְקֶדַח וּבַמּוֹרְדִין. חָזַר הָרֹאשׁ וְהַזָּקָן וְנִקְרְחוּ, הַשְּׁחִין וְהַמִּכְוָה וְהַקֶּדַח וְנַעֲשׂוּ צָרֶבֶת, טְהוֹרִים. הָרֹאשׁ וְהַזָּקָן עַד שֶׁלֹּא הֶעֱלוּ שֵׂעָר, הֶעֱלוּ שֵׂעָר וְנִקְרְחוּ, הַשְּׁחִין וְהַמִּכְוָה וְהַקֶּדַח עַד שֶׁלֹּא נַעֲשׂוּ צָרֶבֶת, נַעֲשׂוּ צָרֶבֶת וְחָיוּ, רַבִּי אֱלִיעֶזֶר בֶּן יַעֲקֹב מְטַמֵּא, שֶׁתְּחִלָּתָן וְסוֹפָן טָמֵא. וַחֲכָמִים מְטַהֲרִים:

Mishnah Negaim 7:2: Negaim [diseased patches on skin, clothes, or houses that create impurity)] that changed their appearance, whether leniently or stringently. How [can it change] leniently? [If] it was like snow and became like the lime of the sanctuary [or] like white wool [or] like the membrane of an egg [ - that is - ] a a Se'et [discolored patch of skin, of the brightest white hue, which, upon determination by a Kohen, renders its bearer leprous and thus severly impure] [becomes like its] derivative, or an Azah [discolored patch of skin, of a secondary white hue, which, upon determination by a Kohen, renders its bearer leprous and thus severly impure] [becomes like its] derivative. How [can it change] stringently? [If] it was like the membrane of an egg and became like white wool [or] like lime in the sanctuary [or] like snow. Rabbi Elazar ben Azaria declares pure. Rabbi Elazar Chisma says: [If it changed] leniently, it is pure. Stringently, it should be checked as if new. Rabbi Akiva says: Whether [it changed] leniently or stringently, it should be checked as if new.

נִשְׁתַּנּוּ מַרְאֵיהֶן, בֵּין לְהָקֵל בֵּין לְהַחְמִיר. כֵּיצַד לְהָקֵל. הָיְתָה כַשֶּׁלֶג וְנַעֲשָׂה כְסִיד הַהֵיכָל כְּצֶמֶר לָבָן וְכִקְרוּם בֵּיצָה, נַעֲשֵׂית מִסְפַּחַת שְׂאֵת, אוֹ מִסְפַּחַת עַזָּה. כֵּיצַד לְהַחְמִיר. הָיְתָה כִקְרוּם בֵּיצָה וְנַעֲשֵׂית כְּצֶמֶר לָבָן כְּסִיד הַהֵיכָל וּכְשֶׁלֶג, רַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה מְטַהֵר. רַבִּי אֶלְעָזָר חִסְמָא אוֹמֵר, לְהָקֵל, טָהוֹר, וּלְהַחְמִיר, תֵּרָאֶה בַתְּחִלָּה. רַבִּי עֲקִיבָא אוֹמֵר, בֵּין לְהָקֵל בֵּין לְהַחְמִיר, תֵּרָאֶה בַתְּחִלָּה: