The original question was about going to a lighter gauge, but assuming we're talking about sticking with strings from the same maker we're also unavoidably talking about going to strings with less tension, and that's something I'm curious about.
Is there a certain 'window' of tension that strings sound good in? For example, take two popular gauges of strings, both tuned EADG. A 40,60,80,100 set and a 45,65,85,105 set. The 40-100 set will be lighter in tension by a few lbs. per string, but it's a popular gauge that a lot of players use so obviously there's nothing 'wrong' with how they sound.
But what if you change the tension more drastically? Regardless of the gauge being used, it's kind of universally agreed that you don't tune down too much because your tone will suffer. In other words, you wouldn't take that same 40-100 set and tune down to BEAD (which would be the same as a 20,40,60,80 set tuned EADG). They'd be too floppy and would sound pretty 'rubber-bandy.' We've all experienced this when we change the strings on our basses, right? You've got your bass plugged into your amp and you start tuning the strings down and they go from sounding normal to having that dull thunk of a sound. Then you put the new strings on and they have the same dull thunk until you get enough tension on them. That's what I mean about having a 'window' or range of tension that strings will sound good in.
But what about going to higher tension? (assuming you adjust your truss rod to compensate for it). This is something I never see brought up. What about a 55,75,95,115 or 60,80,100,120 set tuned EADG? What does having higher tension do to the tone?