brake line opinion

Well, things could go either way. Im all for changing them.
But, moisture settles in the lowest spot.So for me, if theres little to no rust in the rear cylinders, Id be tempted to leave the rear lines. The fronts are tricky. As the line goes down bellow the frame and then back up, it provides a perfect place for water to settle. However, the water requires oxygen to actually rust. So if the reservoir to the front was not empty,preventing oxygen from getting down there,then those lines may be fine also.
Im kinda with Krazy on this one.
I might be tempted to change out just the front loops under the frame.Nah,maybe not.
What you could do is drain all the fluid into a clear mason jar, including any flushing fluid. Then let it sit for a few days, and see if any water settles out.
Of course if youre going racing, your life may depend on those brakes being there for you.
And finally, yes, replace all the soft lines. No question about that.