Officially voiding a contract after both parties agree to void it

Question

Two parties agreed to be bound by a contract after (electronically, not digitally) signing an online contract which both parties have downloaded to their PC.

Now the parties had a good ol' chat and agreed to amend the contract and re-sign. How can the previous contract be "officially" voided? After all, any party could retain a copy and claim it was never voided.

Is it sufficient to keep a log of communication about it?

Hello John,

We both agreed for the contract #123 dated 1 January 1970 to be voided.
Would you be so kind to put that agreement into writing by replying?

How about if the contract was signed in an automated system. Could the system send-out a link to John's email address with a unique link saying the following?

Hello John,

Bob would like to void the contract #123 dated 1 January 1970.

Do you agree? If so, please click this link and then check the box and hit 
"OK, please void" to indicate your agreement.  

(Your IP and timestamp of acceptance will be stored on our servers.)

Are these all fair valid ways of going about it?

Regarding the latter, what if John forwards the email (that contains the entirely unique link only known to him) to someone else who accepts although he didn't want to? What if John claims it wasn't him?

Answer 1

Reaching an agreement (a.k.a. contract) to void a previous agreement is sufficient.

Establishing or proving that the parties reached a valid "agreement to void" is the same problem as proving any other agreement or contract. I.e., the more unambiguous the terms of the agreement and the more unimpeachable the evidence that the parties agreed to them the better. Written and/or recorded evidence of agreement obviously helps. All three tags on this question are worth reading for further details.