I want to change relegated teams from last season into promoted teams, should I edit the players using the creation center or using an editor like FCDB XP? I know i'll have to use FCDB XP to update the kits/flags/badges.

Use FCDB Xp 2, it's betrter and youll have more control editing your rosters..

Or use FEd, both would be fine. But definitely not Creation Centre, there are a lot of limitations.