It’s a multistep process. First, select the clip whose settings you like, and hit command-c (copy). Then select the clips you want to change, and hit shift-command-x (or Edit > Remove Attributes…) and then hold option and select the keyer effect. This will select ONLY the keyed effect that you want to remove. Then hit shift-command-v (or Edit > Paste Attributes) and option-choose the keyer effect again, which will paste ONLY the keyed effect. I’d suggest trying this on a subset or a duplicate set of clips (maybe copy/pasted at the end of the timeline, or in a Snapshot) first, before doing the whole timeline, just do you have a sense of how it works.