The following will link you straight to each sales channel and market places help guides on how to bulk update your SKU codes. 

How to edit listing and product information

Build your inventory file

How to use the new manage inventory

Tip: eBay suggests for merchants with SKU codes to use there Merchant integration platform -  a bulk management tool for high-volume sellers

File exchange and Merchant Integration Platform info

Merchant integration Platform

eBay selling manager pro custom label information

Bulk editing products and variants

Adding and Managing Products

Bulk updating via CSV export and import 

Tip: Magento offers an add-on to bulk update your product information, including SKU code.

Magento bulk product update