Q. On tefilin shel yad, if the last parsha became pasul, can yo cut it off and glue a new correct parsha recently written so it will be kesidrom (in order)?

A. Shulchan Aruch (32: 47) and Mishna Berura (ibid. 219) maintain that on the onset the four parshios of the tefilin shel yad should be written on one single parsha. If they were not, after the fact they should be joined together with kosher glue. If one of the last parshios became disqualified, one could cut that parsha off and glue a new one that was written after the first ones, so the kesidron order will be kept.
Keren Ledovid (8) asserts that one who sells tefilin has to inform a prospective buyer, that the shel yad parshios are glued together, since the value of tefilin with parshios shel yad glued together, may be less than if they were one single parchment.
The above applies only to complete parshios, however one may not glue together the parts of a single parsha. (Piskey Teshuvos 32: 72).
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a