If there's a new tech or improvement available to them I'd say go back and re-do or upgrade. If it's already considered good by majority then keep going forward.
If it's their first project and the development took em years, I would say just revamp them game if feasible, It there was no difference/improvement story or art wise, just make a new game at taht point
In some games you can see the dev improve over the chapters of the VN they make.
Do you prefer they continue making new chapters or improve/upgrade earlier chapters?
Depends. If the game is outdated I appreciate updates to old content with new graphics or animations, otherwise just move on if early chapters were merely an introduction.