This one is not very practical, especially since you need to alter each cell comment(or copy and paste), but still worth noting.

To modify a cell comments autoshape:

Insert a comment
1. Right Click a cell and select “Insert Comment”, type the text you’d like as your comment.

Edit the comments autoshape
1. Right click the cell and select “Edit Comment”
2. Click on the gray square border surrounding the comment, this takes it out of edit mode.
3. On the drawing toolbar select the word “Draw”, this should be on the left most part of the drawing toolbar for most.

4. From the popup select “Change Autoshape”, and pick the new autoshape for your comment.