If you clean your vents, you may dislodge stuff that will go in the rest of the system, so I would do that first, and that way less bother.

Replace the flex first.

Thanks- makes sense!

Need to clean/replace parts of the system. Wondering which order is best. Clean the vents from the rooms first or change out a long 20'+ run of old flexible duct in the attic. I am definitely planning to do both so that is not the question. Just want to get advice on what should be done first.

Thanks!