Additionally, Google will be retiring some functions of the Picasa API.
As for Picasa Web Albums, its users have two choices at their disposal.
Since the update to Stacks 3, updating stacks have changed. This is a nice touch, in case you do not want to update to a stack for some reason.
You have two different choices, you can click the blue Update All button and all of the updates will happen at one time, or you can update the stacks one at a time.
If you want to keep using Picasa, you can absolutely do so.
As of March 15, 2016, Google will no longer be supporting the desktop Picasa application for Mac and Windows.