How do you handle mid-contract upsells in Hubspot?

I'm curious on how other SaaS companies handle the following: a customer has an annual contract, and after 4 months wants to add extra users for the remaining 8 months.

Do you:

1. Create a new deal for the upsell (gives visibility into the revenue step-up), or
2. Amend the existing deal's line items?

How do you handle this in HS and why did you choose that approach?