This will do everything correctly:

find -L . -type f -name "*.so" -print0 | while IFS= read -r -d '' FNAME; do    mv -- "$FNAME" "${FNAME%.so}.dylib"done

By correctly, we mean:

1) It will rename just the file extension (due to use of ${FNAME%.so}.dylib). All the other solutions using ${X/.so/.dylib} are incorrect as they wrongly rename the first occurrence of .so in the filename (e.g. x.so.so is renamed to x.dylib.so, or worse, ./libraries/libTemp.so-1.9.3/libTemp.so is renamed to ./libraries/libTemp.dylib-1.9.3/libTemp.so - an error).

2) It will handle spaces and any other special characters in filenames (except double quotes).

3) It will not change directories or other special files.

4) It will follow symbolic links into subdirectories and links to target files and rename the target file, not the link itself (the default behaviour of find is to process the symbolic link itself, not the file pointed to by the link).

for X in `find . -name "*.so"` do mv $X ${X/.so/.dylib}done

A bash script to rename file extensions generally

  #/bin/bash  find -L . -type f -name '*.'$1 -print0 | while IFS= read -r -d '' file; do      echo "renaming $file to $(basename ${file%.$1}.$2)";      mv -- "$file" "${file%.$1}.$2";  done

Credits to aps2012.

Usage

1. Create a file e.g. called ext-rename (no extension, so you can run it like a command) in e.g. /usr/bin (make sure /usr/bin is added to your $PATH)
2. run ext-rename [ext1] [ext2] anywhere in terminal, where [ext1] is renaming from and [ext2] is renaming to. An example use would be: ext-rename so dylib, which will rename any file with extension .so to same name but with extension .dylib.