Reader-enabled PDFs: no restrictions

No it's not.

The message indicates that once Reader-extended rights have been applied, you cannot make further edits to the PDF in Acrobat without invalidating those rights. Reader-extending a file signs it using a private key, and if the PDF file changes after that point other than in the ways permitted by the key (commenting, form-filling, etc.), the hash calculation for the signature will fail.

Not possible.

The message is for Acrobat users warning them that because of these rights, they will not longer be able to edit the PDF. To do that, they will need to save the PDF as a different file to remove the extended rights.

Reader can never edit a PDF.

Hi Mary,

I'm going to expand a bit on what George mentioned.
1) Open the Reader Enabled file in Acrobat Pro
2) Select the File > Save a Copy menu item
3) Click the Save a Copy button on the alert dialog that warns you are about to remove the Reader extended features
4) Supply a new file name and then click the Save button
5) Open the file you just created (it doesn't open automatically like you get with "Save As")
6) Make your changes
7) Reader Enable the updated file

Steve