Another alternative to copying it in geographical sections is to copy it over based on the auto visgroups.
Depending on how things are laid out, this might let you copy over only a few things at a time but have more possible reference points to help make sure everything is correctly aligned in each step.
Also, I've been working on a symmetric map recently, and I've found it saves time to use custom visgroups to break apart my BLU side into normal stuff (Brushes, func_nobuild) and team-specific stuff (spawns, triggers, doornames.)
That way I just make a single flipped version for the red spawn/flag/doors, and just hide it and maintain that separately from the regular geometry. It's easier than going back and repeatedly re-renaming all the I/O links etc.