HBO CEO Richard Plepler is out amid a broad restructuring of the company under new parent AT&T, according to an internal memo obtained by CNBC.

Plepler had been at HBO for nearly three decades and led the company as it became one of the market leaders in streaming video on demand. AT&T's purchase of Time Warner earlier this year spurred reports of a culture clash between the telecom giant and its newly acquired media assets.

Shortly after the merger closed, media reports pitted the head of the newly named Warner Media, John Stankey, against HBO incumbents, saying Stankey was angling for larger content volume out of the company.

"Hard as it is to think about leaving the company I love, and the people I love in it, it is the right time for me to do so," Plepler said in the memo. "In the past weeks, I've thought a lot about the incredible journey of this company in the nearly 28 years that I have been blessed to be here."

In a statement to CNBC, Stankey said, "Richard is one of the most successful executives in our industry and I have been fortunate to have his support over the last months. His vision, energy and passion helped to elevate HBO's brand to what it has become today. Richard's impact to our business and on the passionate viewers of HBO's enduring programming will continue to be felt for years."

Plepler is leaving in part because he would have seen less autonomy under the new structure, according to a person familiar with the transition. Plepler is still working out the exact timing of the transition, the person said, but chafed at the prospect of a larger company and more oversight.

It's notable timing for such an executive exit. HBO is increasingly facing competition in the premium content streaming space and in original programming. Streaming companies like Netflix and Hulu and media giants like Disney and Comcast-owned NBCUniversal are all eyeing market share.

AT&T is planning a new, separate streaming service anchored by HBO content to launch later this year.

Here's Plepler's memo: