This Agreement constitutes the full understanding of the parties and supersedes all prior agreements, understandings and proposals, whether written or oral, with respect to the Work. No modification of this Agreement shall be binding unless contained in a writing signed by both parties. Subject to applicable statutes of limitations, no waiver by either party of any breach or default hereunder shall be deemed a waiver of any future breach or default or shall in any other way affect any of the terms or conditions hereof.
Most contracts contain this provision, which simply stipulates that any subsequent modifications to the agreement must be in writing and signed by both parties in order to be legally valid. This requirement is especially important if your deadline for submission of the manuscript is extended (be sure you get the date change in writing). It is best to confirm by a letter with a line for each party's signature.