I have three identical Zope 2 instances (2.13.26, python 2.7.12, linux2) that have unfortunately grown quite large without my noticing it. Packing any of these will exceed the space on disk due to the creation of Data.fs.old. Adding disk space through my hosting provider is irreversible and so too expensive to be a good strategy.
On the other hand, since these Zopes are identical, I can risk packing without creation of a backup. Is that possible? I don't mind tinkering with the source for this purpose.
You write about setting pack-keep-old false in zope conf for the filestorage?
I would create a recent off-site backup (I hope you have a nightly anyway) and then pack it. I never had any problem packing a DB which resulted in the need of restoring a .old files, but better safe than sorry.