Is there a shortcut to switch between sentry and observer ward when quickcasting?

Double tap the 'cast' item hotkey you would press to use a ward

The default hotkeys Z,X,C,V,B,N based on the item slot (left to right, top row first)

A quickcast key cannot be used in this method. However, if you are mapping all items to quickcast, a solution is to set an alternate key combo for plain 'cast'. I recommend ALT+key. Then you can hold alt and double tap the key.

For example

quickcast item 1 = z

cast item 1 = ALT+z

With this holding ALT and double tapping z will swap wards, without impacting quickcast.

Wiki source