My expected answer is no, but wanted to make sure about it.
Hi Samet,
Your expected answer is correct: no automatic switch. SLDS 2 (via the Salesforce Cosmos theme) requires an admin to explicitly activate it in Themes and Branding — Salesforce doesn't flip production orgs onto it automatically as part of a release upgrade. This has been the case since SLDS 2 went GA in Winter '26 and remains true for Winter '27.
What is new in Winter '27 specifically: group-based SLDS 2 activation — admins can now activate Cosmos for specific segments of users rather than only an all-or-nothing org-wide switch. That's a rollout-control improvement, not a change to the "opt-in" nature of the activation itself.
One nuance worth flagging separately: Flow Builder's canvas UI itself is being updated to the SLDS 2 Cosmos look in Winter '27 as part of the tool's own interface — that's a Salesforce-authored admin tool getting a visual refresh, not your org's Lightning Experience theme being switched.
References:
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https://www.salesforce.com/blog/experience-design-with-slds-2/-
https://trailhead.salesforce.com/content/learn/modules/salesforce-lightning-design-system-2-for-admins/plan-your-transition-from-slds-1-to-slds-2
