When Guardians of the Galaxy Vol. 3 was delayed, there would obviously be some changes to the movie that originally was meant to kick off Phase 4, but none of those changes will be caused by what happened in Avengers: Endgame.
James Gunn did a fan Q&A on his Instagram (via Comic Book Movie), and he explains he wrote Vol. 3 already knowing what would happen in Endgame:
“The script will change because my scripts always keep changing up until the moment I shoot them.But it won’t change according to anything from Endgame, because I already knew what was in that script before I wrote Guardians 3.”
James Gunn is currently filming The Suicide Squad, and he has said in the past that production on Vol. 3 wouldn’t start until that movie is edited (not done filming). With the movie due in August of 2021, that puts a summer 2022 release for Guardians Vol. 3 in question; meaning 2023 looks a lot more likely.