I have two pairs of Rashi tefillin. Is it permissible to take one of the Shel Rosh of one of the pairs and convert it from a Rashi to a Rabbeinu Tam Shel Rosh. I will then have a pair of Rashi tefillin and a pair of Rabbeinu Tam tefillin. I don’t plan to put on Rabbeinu Tam everyday, but I may on the odd occasion put them on during Torah study in private at home. Please let me know if it is permitted by Halacha, and if not the reason. Thanks

Shalom, Thank you for your question. The changing of tefillin from Rashi to Rabbainu Tam is not allowed – see the Mishna Brurah (34, MB(20)). The reason for this is that it is forbidden to lower the status of a religious article. As the tefillin of Rabainu Tam are on a lower level of holiness than those of Rashi, taking the Tefillin, or even the straps (or even the protective boxes used to store the tefillin in) and now using them for a lower level of mitzvah is forbidden. Interestingly there is some debate in the Kabbalistic sources as to whether the Rashi or Rabainu Tam tefillin are more holy. Because of this debate we in practice do not even change Rabainu Tam tefillin to Rashi (which we might have thought was raising them up to a higher level and permitted). (As the Mishna Brurah ibid points out). I hope this is of some help - Blessings.