Rainbowchaser has it right.
You fight the fish with the rod so going down in line weight will NOT make the fish "feel" any differently. BUT it will make it more difficult to make shorter casts. On casts shorter than about 45 ft (9 ft rod + 30 ft line + 9 ft leader) you will have underweighted the 5 wt rods with the lighter lines.
Beginners generally do better with heavier lines because they can feel the rod bend more easily. Unless you are a proficient caster, I think you will find the lighter line more difficult to cast. So I would not go out and buy a line.
From what you have said about your outfit being too heavy for the fish you are catching, this is what I would do. I would not go to a lighter line. I would actually go to a lighter rod and use the same line. Try to borrow a lighter rod such as a 4 wt and put your 5 wt line on it.
Regards,
Silver
"Discovery consists of seeing what everybody has seen and thinking what nobody has thought"..........Szent-Gyorgy