I need to create a new company template for a new client, and I'd like to create it with an aspect ratio of 16:9. However, all their current presentations are in 4:3 and they need to be able to repurpose slides into the new template quickly and easily. Additionally, there are many offices worldwide that use other sizes (A4) and there's no way to ensure everyone gets onboard with the new template. Many users are using PPT 2010, and some use a Mac.

Any ideas on how I can help provide a modern template while maintaining ease of use?