Join Date: Dec 2004
Location: Altofts zx-12r B4 UK
I would think it depends on how old and used your OE ones are and are they dirtying up your brake fluid.
I changed mine earlier this year after 45k+ miles, but I was stripping the calipers at the time, so thought I may as well fit new brake lines, so new fluid also. Pleased I did.
For Tech Questions Bike Model letters and Country are more useful than saying the year, for UK owners anyway, as registration year is not always the year of manufacturer model.