Others stretch in warm water. In as many colors as jelly beans. Y ou need to match the type of fish, fly, and water you fish to know which line is best. N ow you realize you may wish to use more than one fly line. The reel manufacturers have thought of that.
Most reel companies sell extra spools, cassettes or reel inserts.
Just so you can change lines to fit the fishing du jour. M y personal advice to our casting class students is to buy one very good quality line to start with. If your interest in fly fishing was sparked by the traditional image of dry flies, go for either a double taper or weight forward line. Yes, good fly lines are expensive. But the bottom line is a cheap line will not cast well on any rod.