Thank you for sharing this workaround. Unfortunately, it isn't a practical solution for existing task lists. The only way to restore the square checkboxes is to select all tasks and press Ctrl+1 three times, which is simply not feasible if you have a large number of notes.
Another issue is that you can now only choose between green and yellow square checkboxes. The classic blue checkbox, which had been the default for many years, is gone.
And I honestly have to ask: Why, Microsoft? To-do lists have used square checkboxes ever since Schedule+. It's a well-established visual convention that users instantly recognize.
In my opinion, this is not just a design change—it has a real impact on productivity. The new circles look far too similar to the bullets in a bulleted list, making it much harder to distinguish tasks at a glance.
If you have dozens of notebooks with hundreds of pages, as I do, this seemingly small change becomes a significant usability issue. It breaks years of visual muscle memory and makes it noticeably more difficult to scan notes for open tasks.