The SEC provided an updated timeline on April 20 to provide extensions for CAT target go-live dates. https://www.sec.gov/news/press-release/2020-92
Here is an updated timeline. In summary, go-live dates that were originally planned for early 2020 have been moved back 2 months but subsequent dates remain unchanged.

For those already with OATS reporting obligations, the effort to comply would be to map existing information already reported on OATS to the CAT requirements. One important thing to note is that there is no timeline to shutoff OATS reporting. So, firms must also ensure that the reported information between OATS and CAT are consistent in terms of completeness, accuracy and timelines of data.
For those broker dealers who are currently exempt from OATS reporting, it’s important to note that there are requirements for thee firms to also provide order information by end of 2021. This would require some planning on the part of the smaller firms who are likely already thing on resources.
For some ideas on how to appropriately plan for this, my colleague, George Mandl and I recently provided a more detail discussion of the CAT requirements on this webinar https://vimeo.com/389131349.