Question:
Can I put back a defrosted challah in the freezer on shabbos if I want to preserve its freshness for another time, i thought I would need it for this week but ended up being extra.
Answer:
It is permitted, and it is not an issue of molid, because it is staying a solid. It is also not an issue of hachana,because you are doing it to preserve its freshness, and to keep its present state.
Best Wishes
Sources:
Shmiras Shabbos Khilchoso 12-3.