Variation engagement letter — adding scope mid-engagement

I'm mid-engagement with an existing client and we've agreed to add to the scope. Draft a short variation letter to formalise it (rather than a brand-new engagement letter).

Firm: {{firm_name}}
Client: {{client_name}}
Existing engagement reference: {{existing_engagement_ref}}
What we're adding to scope: {{added_scope}}
Why it's being added (1 line): {{added_reason}}
Additional fee: {{added_fee}}
New estimated completion date for the whole engagement: {{new_completion}}
Any change to assumptions / dependencies: {{assumption_changes}}

Return a 1-page variation letter with these sections:
1. Reference to the original engagement (date + ref).
2. Variation: what is added, briefly.
3. Fees: additional fee + total updated fee.
4. Timeline: any change to delivery date.
5. All other terms from the original engagement letter remain unchanged (one line).
6. Acceptance — sign and return.

Formal but warm. AU English. Mark anything that needs my standard-template review with [REVIEW: X].