Is there a way to change all the font on all the slides at once?

There is no way to do it in one step and it also depends on the way the object styles were used, did you use a custom theme or not? When consulting I have seen a lot of users who do override the object styles and that makes changing to another style a lot more difficult. You have to identify all the object styles that are used in the original configuration (you talk about a 'template', maybe you meant a 'theme'?). You can check the master slides to look for the default object styles. And then go in the Object Style Manager, change those default styles and they will be applied immediately to all objects that use those default styles.

Hi there

Try clicking Edit > Object Styles Manager and see if that offers help.

Cheers... Rick