Rashi explains the gemara's question "Be concerned that he changed his mind" to be that the was mevatel the gett and then decided to continue writing it. Tosafos points out that this implies that the bitul to the gett is effective, which seems to contradict R' Nachman who says that when one is mevatel a gett, if he later decided to use it, the gett itself was not batul and can be used? Tosafos answers that after it is written and is a complete entity the bitul will not be effective, but until it is completed the bitul will effectively nullify the gett.
The Rambam (Hil. Tzitzis 1:15) writes that when one puts tzitzis on a garment that already has tzitzis on it, but his intent is to be mevatel the original ones, and use the ones that he is tying on now for the mitzvah, his bitul is effective to remove the mitzvah status from the original ones. The Sha'ages Aryeh (30) is troubled by this - כיון שאין בהן שום פסול, כי מבטלן איהו מאי הוי, וכי ספר תורה או תפילין ומזוזה שנכתבו בהכשר כדינם וכי אפשר להוציאם להכשירן ע"י ביטול ויהיו נפסלין ומכאן ולהבא ע"י שביטלן, הא לא שמענו, ואי איתא להא מילתא לא אישתמיט שום תנא או אמורא להשמיענו חידוש גדול כזה
To answer this question the Sha'ages Aryeh suggests that the Rambam goes l'shitaso in Hilchos Gittin (6:21)that when the husband is mevatel the gett, it can never be used again - it is like חרס הנשבר "broken pottery". Just as the Rambam holds bitul works by a gett, he holds l'shitaso that it works by mitzvos, but most poskim disagree with the Rambam that bitul doesn't nullify the gett from ever being used again, and similarly hold that it doesn't work by mitzvos.
The Sha'ages Aryeh tries to push off the Rambam as some radical opinion that bitul helps by gett and by mitzvos, but all other poskim would disagree with the Rambam in principal and maintain that bitul is not at all effective. However, based on Rashi and Tosafos that bitul would be effective prior to the completion of the gett, they should also maintain (based on the equation of the sha'ages aryeh, which needless to say is not muchrach) that bitul would be effective to a half written sefer torah or half written mezuza. If this is true, then the majority of poskim do not disagree with the Rambam in principal, they just hold that after it is completed it is not susceptible to being nullified. But in principal they would also agree that one can "nullify" mitzvos and successfully disqualify a half written mezuza from being a kasher mezuza even after it is completed!