Have you checked the relief yet? Going from medium to lights will produce to opportunities for buzz. The first is that with lighter strings and less tension to tune them, the headstock takes away relief by moving away from the bridge towards the back of the guitar. In other words, the headstock is fighting against the string tension too much and is 'winning' by actually making the fretboard hump up in the middle, around the 9th - 12th frets. To remove the hump and add relief you'd need to loosen the trussrod nut. This is an easy adjustment to make. With the guitar tuned to pitch even, FULLY INSERT the wrench into the adjustment nut and turn it counterclockwise about 1/4 turn, say from 9 o'clock to 6 o'clock. Retune your guitar and check it out.
I suppose I should have started with explaining how to check your adjustment. With a capo or a friend with a hand, capo/depress the 6th string at the 1st fret. Use a finger to fret the 6th string at about the 15th fret. Now, see if you can
slide 2 business cards between the 6th string and the 7th fret. Do the cards pick the string up? You need more relief. Do the cards slop around and don't even touch the string? You could take relief away.
Ok, so I've explained how to check and add relief. The other cause for the buzz might be the lighter strings themselves. The lighter the string the less tension there is needed to bring it to pitch. Lower tension = a larger vibration arc. Larger vibration arc = more distance needed away from the frets to minimize buzzing. Smaller strings are easier to press down, but they have to be set a bit higher than heavier strings so they don't buzz. At a certain point, the distance to push down can be just as much, if more, pressure required to fret a note than a heavier string that sits closer to the fret. It's a trade off.