That would just be… ridiculous. Silvermoon is a Horde capital. That’s not likely to ever change, not in game, nor out.
And even if there may be some citizens of Silvermoon disgruntled with being part of the Horde, the fact is that the Blood Elves have been members of the Horde for years now. I would imagine that most of the citizenry have grown accustomed to being part of the Horde after all this time, and any who aren’t happy now have an easy “in” to the Alliance through the Ren’dorei if they really want to change allegiance.