There are lots of variables here. I'd say the one that stands out most is that you probably have a tremolo on your M-II, an OFR or a Sinclair. Assuming you haven't blocked it, you lost a little string tension when you detuned. I don't know if you tightened your springs to compensate, and maybe overshot. Or did you make height adjustments? Another thing that seems obvious but people overlook it, do you have the nut locked down again once you are in tune? Sometimes loose nut clamps rattle like crazy with certain frequencies.

    I will echo the comments of CM86. You may want to go up a gauge in strings, and you will want to raise your action until the buzz goes away (assuming it's not something else). The change in tension could also be affecting your truss rod, and creating a spot where your B/C string hits a fretwire. I have a specific method for truss rod that involves tools and gauges, but in general terms, you have less string tension, so you may want to loosen the truss rod just a smidge. But I'd hold off on that if you're going to step up a gauge, it might equalize the difference.

    Sorry it sounds complicated. This is why most de-tuned players sell their floyd guitars to guys like me, and then go buy fixed bridge guitars. :)