Русско-англо-русский юридический словарь

Supplemental agreements are legally binding documents that are used to modify contracts that are already in force. This type of document is sometimes used as a means of allowing the existing agreement to remain in place with the same ending date, while adding or removing some provisions or terms to the working relationship. A supplemental agreement is often an ideal solution when there is no desire to renegotiate an entirely new contract to take the place of the current agreement. (WiseGEEK)

supplemental agreement Mutually consented agreement that modifies (but does not replace) an existing agreement (BusinessDictionary.com)

You certainly do not have to agree to change the completion date to suit your buyers, but if you do, it is very important you tell your solicitor so he can then vary the contract formally.

Ideally, your solicitor should prepare a supplemental agreement changing the completion date and your buyers should pay your solicitor's costs for drafting it. Both you and the buyers will need to sign the supplemental agreement.