I have typeset a few book chapters already, but I chose to enlarge the letters, which lead to the text needing more pages. That led to the text frames being uncentered. Is there a way to auto-centered the text frames, with a different option for left and right pages?
sounds like a job for "Master Pages". You specify how left and right pages should be set and save those settings as a master page.. Then just click to apply to pages as needed.
Take a look here for more info and see if this works for you.
https://wiki.scribus.net/wiki/index.php?search=master+pages&title=Special%3ASearch&go=Go
What I meant was, although I have made master pages, that I have placed text frames in them and I added a page between chapter 8 and 9. Now, all chapter 9 main body text frames are slightly of center for their respective pages (a little bit to the right for the left pages and a little to the left for the right ones). What I am searching for is a way for the added text frames to automatically center themselves based on if the page is right or left. (And still be able to move them, if needed).
You need a left and a right master page, and apply them to the corresponding pages.
Sounds like you need to use the 'Apply Master Page' dialogue to reapply the correct master pages from where you added the page between chapters 8 and 9.