Views: How to change entity view mode settings when a contextual filter is available?
Just create a view with two page displays. 

The first display will be on the path with no arguments and will display teasers.

The second display will be on the same path with an argument such as thepath/% and will display full view mode. 

When you alter the format and add the contextual filter in the second page, make sure to select it only for that display, otherwise it will overwrite the first display as well.



This is what views displays are for. Use it.