The warning message tells you that the package doesn't know how to automatically upgrade the materials that your existing sprites are using. So, it's up to you to change their material to the one you want.

If you have a sprite that's already behaving the way you want, take the material it's using and apply it to the others.

If not, you can create a new material and set its shader to `Universal Render Pipeline > 2D > Sprite-Lit-Default`

Then select each sprite that's not behaving, and set it to use this new material.