Sara Lance has become one of the longest-serving and most popular characters in the Crisis on Infinite Earths. The early days of Oliver's vigilante journey, however, were a far cry from what they would become. Returning home after five years away, Oliver operated as The Hood with no allies and with absolutely no qualms about snapping people's necks. Over time, he embraced more heroic methods and a system known colloquially as Team Arrow. One of the earliest was the first Arrowverse incarnation of Black Canary.
It's safe to say that the role is now synonymous with Caity Lotz. Following an uncredited appearance in Arrow season 2, episode 1, "City of Heroes", Lotz made a direct debut two episodes later. She recurred throughout season 2, ing Team Arrow and briefly reigniting her romance with Oliver. To secure back-up in the fight against Slade Wilson, however, Sara was forced to reaffirm her commitment to the John Constantine, helped restore her soul, Lotz became the White Canary and made the jump to the ensemble spinoff.
When Arrow first premiered, Sara Lance was played by a wholly different actress: Jacqueline MacInnes Wood. Featured exclusively in the pilot's flashback sequences, Wood played Sara during her initial, illicit fling with Oliver. Traveling with Oliver on the Queen's Gambit, she seemingly went down with the ship when it sank due to sabotage. Though that was famously not the case, it marked Wood's only episode on the show before Lotz came on board to take over.
The reasons for the recasting have little to nothing to do with Wood's suitability for the role or general performance. Instead, it's more to do with availability. Wood has had several parts over the years - on screens both big and small. As well as starring in Castle. The operative word is guest-starred. In 2008, Wood debuted as Steffy Forrester on The Bold and the Beautiful. Having starred in over 1,500 episodes, Wood has retained the role ever since. The Arrowverse producers no doubt conceived Sara Lance as a one-off character, making it compatible with Wood's no doubt hectic schedule. Once they changed their mind, however, it would have been nigh impossible for her to juggle both commitments.
Given that Sara Lance would take on a more active role in Arrow season 2, it was likely also beneficial to find an actress more experienced with action sequences and stunts. As a skilled martial artist, Lotz more than fit the bill in that regard. Though it's unknown whether Wood laments missing out on such an eclectic role, it was the right decision all around. Even when Lotz was cast, the character was meant to have a short shelf-life. Like top billing on Legends of Tomorrow. That is a long way to have come since Arrow.