Even though it's always best to configure color management when you first start a new project or scene, you may find it necessary to apply color management to existing scenes that do not use it yet.
If the scene was saved in Maya 2015 or a previous version, see Compatibility with color management in older versions for information about how the legacy color profiles are converted to color spaces.
If there are many nodes to change, it is probably quicker to define and reapply the color space input rules. See Change existing input color spaces automatically by reapplying rules. However if the scene uses image-based lighting (IBL), you will still need to update the Color Space of the mentalrayIblShape node manually.