If you try to write a file backing an executable mapping you get ETXTBSY (or not, if it's NFS/some other broken FS). So what everyone is doing is unlink the old file and create a new one. Unlinking preserves open file descriptors, extending to mappings.

Note that often the actual backing files are disjunct anyway and only symlinks get replaced, due to versioned .so names (as in libfoo.so.2.4.5, libfoo.so.2.4@, libfoo.so.2@, libfoo.so@, you've all seen it).