Why was Lady Laena actress replaced in House of the Dragon?

In the Targaryen history book, Laena and Daemon have twins daughters Baela and Rhaena Targaryen.

Rhaenyra and Laena became good friends after the Velaryon lady married Daemon, who was also her sister-in-law through her marriage to brother Laenor (Nate).

Daemon and Laena would often visit Rhaenyra on Dragonstone with the twins betrothed to Rhaenyra’s sons, showing unity between the two families.

Sadly, Laena died in childbirth after her third was stillborn and she was struck by grief and a fever after a prolonged labour.

Whether the show follows this trajectory remains to be seen with the show already diverging from the source material.

House of the Dragon airs on HBO on Sundays and Sky Atlantic on Mondays and streams on NOW 

source: express.co.uk