אורח חיים מ״בOrach Chayim 42

Se’if 1א׳

אָסוּר לְשַׁנּוֹת תְּפִלִּין שֶׁל רֹאשׁ לַעֲשׂוֹתָן שֶׁל יָד, וַאֲפִלּוּ לִקַּח רְצוּעָה מֵהֶם וְלִתֵּן בְּשֶׁל יָד אָסוּר, מִפְּנֵי שֶׁאֵין מוֹרִידִין מִקְּדֻשָּׁה חֲמוּרָה לִקְדֻשָּׁה קַלָּה, וְשֶׁל רֹאשׁ קְדֻשָּׁתוֹ חֲמוּרָה, שֶׁרֻבּוֹ שֶׁל שַׁדַּי בְּשֶׁל רֹאשׁ, אֲבָל מִשֶּׁל יָד לְשֶׁל רֹאשׁ מֻתָּר לְשַׁנּוֹת, וְאִם הָיוּ חֲדָשִׁים שֶׁעֲדַיִן לֹא הֱנִיחָם, מֻתָּר לְשַׁנּוֹת אֲפִלּוּ מִשֶּׁל רֹאשׁ לְשֶׁל יָד, (שֶׁטּוֹלֶה) [שֶׁתּוֹלֶה] עֲלֵיהֶם מִכְסֵה עוֹר אַחֵר, וְנִרְאִים כְּבַיִת אֶחָד.

Laws regarding whether it is permitted to change hand תפילין into head תפילין. And there are 3 Se'ifim: It is forbidden to change head תפילין and make them into hand תפילין. And even to take a strap from them and place them on the hand תפילין is forbidden, since we don't lower [an item] from a stricter (ie. higher) sanctity to a [more] lenient sanctity... and the head תפילין has a stricter sanctity since the majority of the [Holy Name] שד-י is on the head תפילין (ie. the ש on the box and the ד on the back of the head straps). But [to change] from hand תפילין to head תפילין is permitted. And if they were brand new and were never worn, it is permitted to change them... even from head תפילין to hand תפילין... for he affixes on them (ie. on the head תפילין box which has four visible compartments) a single leather covering and then they appear as one compartment.

Se’if 2ב׳

אִם הִתְנָה עֲלֵיהֶם מִתְּחִלָּה, אֲפִלּוּ לְבָשָׁן אָדָם, יָכוֹל לְשַׁנּוֹתָן, אֲפִלּוּ מִשֶּׁל רֹאשׁ לְשֶׁל יָד.

If he made a stipulation on them (ie. on the head תפילין) originally [when he made them], [then] even if someone wore them he may change them... even going from head תפילין to hand תפילין.

Se’if 3ג׳

סוּדַר דְּאַזְמְנֵהּ לְמֵצַר בֵּיהּ תְּפִלִּין לְעוֹלָם, וְצַר בֵּיהּ תְּפִלִּין חַד זִמְנָא, אָסוּר לְמֵצַר בֵּיהּ זוּזֵי: הַגָּה: וְאִם הִתְנָה עָלָיו מִתְּחִלָּה בְּכָל עִנְיָן שָׁרֵי (בֵּית יוֹסֵף בְּשֵׁם ר' יְרוּחָם ומהרי"א) וּקְלָף הַמְעֻבָּד לְשֵׁם תְּפִלִּין אָסוּר לִכְתֹּב עָלָיו דִּבְרֵי חֹל, דְּהַזְמָנָה כִּי הַאי גַּוְנָא לְגוּף הַקְּדֻשָּׁה מִלְּתָא הִיא (רַ"ן ס"פ נִגְמַר הַדִּין) אַזְמְנֵהּ וְלֹא צַר בֵּיהּ, אוֹ צַר בֵּיהּ וְלֹא אַזְמְנֵהּ, שָׁרֵי לְמֵצַר בֵּיהּ זוּזֵי:

A kerchief that one designated to wrap his תפילין in always (ie. as it's permanent pouch), and he [actually] wrapped his תפילין in them one time... it is forbidden to wrap money in them. Rem"a: And if he stipulated on them (ie. on the kerchief, that he wants to be able to use it for non-sacred items) originally [when he took the kerchief], then it is permitted for all use. (Beit Yosef in the name of R. Yerucham and Mahar"i). [Regarding] parchment that was processed for the sake of [the mitzvah of] תפילין... it is forbidden to write mundane things on them because designation [done] in such a way [where it involves] the sacred item itself is [considered] significant (Ra"n at the end of the chapter "When the trial has ended" - ie. Tractate Sanhedrin chapter 6). If one designated [the kerchief] but did not [yet] wrap them (ie. the תפילין) [in it], or he wrapped them but did not designate it... it is permitted to wrap his money in it.