UK sole trader · extra work
Put the change on the page. One page: what changed, why, the price, the date, two signatures. Free to use here. For the full printable, see the Quote Template. Print forever.
This page is free. The full template: £4.99
This variation is agreed. Work on the change starts when both signatures are on the page.
Not legal advice. Drafts to check. Only vary what your terms allow.