decking has a "shelf life" of 10 years if its older then replace simple, if its younger then you may and i say "may" depending on supplier have it under warranty as treated timber from places like travis perkins or jewsons have a 10 yr guarantee. only problem is if you cut the timber you invalidate the warranty as your exposing untreated timber. but if you have full lengths then why not try your luck.
If you have read the instuctions then any exposed ends after cutting should be treated, if you do this then the warranty is not invalidated. Any decent installer will do this to keep their reputation.
As for cleaning the decking prior to re treating, get some jeyes fluid spray it with that then scrub with a bass broom, leave to dry then recoat with the product of your choice.