• See Projectile notes for more on this package.
• See my StackOverflow question on this.

projectile-replace

• Runs interactive query-replace on all files in the projects.
• See Projectile docs on interactive commands.

In spacemacs:

• Use SPC p R
• Then SPC f S (evil-write-all) to save the buffers.

• SPC s p
• C-c C-e (helm-ag-edit)
• SPC s e (iedit-mode)
• C-c C-c to commit that changes