Hi sarahr202,
Greetings If am getting your question right, it will survive a reboot if you store the Junos OS file in the /var/tmp ( actually any file) if you use the request system software add + Junos OS file +reboot = upgrade, the file will be pushed to the file system from the /var/tmp/ directory.
If this solves your problem, please mark this post as "Accepted Solution" so we can help others too \:)/
Regards,
Lil Dexx
JNCIE-ENT#863, 3X JNCIP-[SP-ENT-DC], 4X JNCIA [cloud-DevOps-Junos-Design], Champions Ingenius, SSYB