The uniqueness or presence of the integer “id” field in activities will not be enforced with the rollout of Marketo's next-generation Activity Service beginning in May 2017. An additional string field called “marketoGUID” is being added. “marketoGUID” will be considered the unique ID for retrieved activities when it becomes available.
Cutover of this change will begin to affect subscriptions with the Spring 2017 release, currently planned to begin May 17th 2017.
If we query for the activities dated on May 15th 2017 on May 18th 2017 will the activities have "id" field or "maeketoGUID" or both?
You should be able to find this answer in these existing discussions: