Veeva has heard from customers that Salesforce’s Lightning Experience Critical Update announcement yesterday makes it appear as though every user will be impacted by this update. Veeva wants to clarify that only users who already have the Lightning Experience enabled for them will be directly impacted. Please note that Veeva CRM is delivered with Veeva custom profiles, however, standard profiles such as System Administrator already have the Lightning Experience User permission enabled.
Salesforce will be turning on the Lightning Experience on a rolling basis starting in Winter '20 in all orgs. Turning on the Lightning Experience in your org will not impact your users unless the individual user has the Lightning Experience User permission enabled via profile or permission set.
We are working hard to make Veeva CRM Lightning Ready for our customers. We will have more details in early 2019.
Salesforce Release Note: https://releasenotes.docs.salesforce.com/en-us/spring19/release-notes/rn_cruc_lex_turned_on.htm
Salesforce Knowledge Article: https://help.salesforce.com/articleView?id=000273853&language=en_US&type=1