Enjoy!
This is my theory! H2H went through many lineup changes before debut!
The original lineup rumors included Yuni (02 trainee), Helen (03), Ouju (04), Soyoon (05) and Sherlisa (05 and still with SM) and Na Haeun!
My first theory is SM picked Ian over Na Haeun! Na Haeun was rumored to be the center for a long time. Then rumors went around the new trainee Ian looked like Na Haeun. During SM trainee showcase people were surprised Na Haeun wasn’t center position and hardly performed. After the trainee showcase she left. Imo SM had picked Ian to be the new center of H2H taking Na Haeun and other candidates place 
My next theory is Stella entering the lineup changed the group members. Stella confirmed was the last member to join SM! She didn’t participate in SM showcase in 2023 and joined after. She mentioned she has the shortest trainee period in the group of under a year. My theory is Stella took Ouju spot
Ouju was an original rumored member since aespa trainee days! She was rumored to be the main visual of aespa and then H2H and eventually the leader of H2H! I think once Stella was recruited they went with Stella as she is also a visual, speaks English and has a lead/main vocal position.
By now only Yuha, Jiwoo, Juun, Yuna/Ana and Ian were set members!
Ouju was rumored to be a visual/dancer/Sub vocal position. People predicted Ouju would debut as recent as January so her not debuting was a shock and people even thought Stella was Ouju!
SM acted quickly to get Stella and her teachers/instructors advised her to go with SM!
Last theory 
Yuni was almost going to debut with aespa but didn’t make it! After she left SM but SM asked her to comeback with a debut offer for H2H this is supported with H2H predebut pictures that include Yuni! She also took part in the 2023 trainee showcase.
Along with Yuni there was Soyoon and Sherlisa in the debut lineup! It was rumored Sherlisa was dropped from H2H at the very last minute as there were originally 9 girls fans are guessing SM is saving her for the new gg!
My theory is Yeon who has the second shortest trainee period after Stella of just over 1 year was made an official member after SM showcase. SM probably didn’t want a group with a large age gap from 02 - 2010 so they had decided to not move forward with Yuni! Soyoon was also let go as they now wanted Jiwoo as the leader with the age gap from 2006 to 2010 lineup
By now The full lineup was set and leaked pictures and videos started appearing predebut H2H!
Part 2 is coming shortly
