If someone has 2 pairs of tefillin, one rashi and another rabbeinu tam. The person always uses the rashi pair. The rashi pair is being sent to be checked or repaired. The Rabbeinu tam pair has been checked and is 100% kosher for use and bracha. The rabbeinu tam pair is never used. Can a person use the rabanu tam pair now and be yozei tefilin, or does he need to seek out a rashi pair?
Even for a short time, one must ensure to wear Rashi tefillin every day, and you should arrange for a replacement in the interim. The custom is not to recite a berachah on Rabbeinu Tam tefillin, and because the principle halachah follows Rashi, one must ensure to wear Rashi tefillin daily.