Although 'Lord of the Rings' may bear the same name as the movie series, the main cast announcement featured exactly zero members of the movie's cast.
In fact, the casting announcement for Amazon's 'Lord of the Rings' was pretty light on details, aside from the names of the actors and not much else. So far, only one actor has their character's name confirmed, and from what we're able to deduce, it's a completely new and original character.
Amazon confirmed the names of Robert Aramayo, Owain Arthur, Nazanin Boniadi, Tom Budge, Morfydd Clark, Ismael Cruz Córdova, Ema Horvath, Markella Kavenagh, Joseph Mawle, Tyroe Muhafidin, Sophia Nomvete, Megan Richards, Dylan Smith, and Charlie Vickers. Recognise any of those names?
Well, let's start at the one you most likely would have seen - Robert Armayo. He played young Ned Stark in 'Game of Thrones', in the Tower of Joy flashback scene. Owain Arthur, meanwhile, was in 'Babylon' as Nobbo. Nazanin Boniadi had a lead role in the excellent / underrated TV series 'Counterpart', opposite JK Simmons. Tom Budge was in Steven Spielberg's follow-up to 'Band of Brothers', 'The Pacific', which also starred a pre-fame Rami Malek. Morfydd Clark played Mina in the recent BBC three-part 'Dracula' miniseries, while Ismael Cruz Córdova was in 'The Mandalorian'.
Sensing a pattern here? Exactly, they're all upcoming actors in the same way that the original cast of 'Lord of the Rings' were. Viggo Mortensen, Orlando Bloom, Elijah Wood, Sean Astin - these guys weren't exactly household names when they started work on the original movie, so the same is true for the TV series.