Muramasa doesn't count as a canon, so its a void transformation. There is also the fact that the transformation that Ichigo had in Las Noches was significantly different in appearance than his previous transformation with the Vizards. And you're making assumptions on the other Vizards that they haven't transformed before. They had to go through the same process that Ichigo did when was fighting to submit his hollow. But there was over a century of time that is technically unaccounted for. I don't think it has anything to do with numbers of times that you do a full transformation. As I said before I think it more based on the change in his powers or increase in his powers.

But here is an interesting question to pose. It could be something that we could tackle in our RP. We've seen through Ichigo's adventures that he struggles with his control over his hollow. The hollow is all about instinct and power in a lot of cases. It is view almost as an animalistic trait. And in general hollows have masks and hard shells/skin. But we've never really seen what its like for hollows and arrancar, especially arrancar when they are force to do battle over their shinigami parts. Arrancar have gained access to shinigami powers and while shinigamis are different from hollows. There has to be this same sort of subconscious power struggle that Arrancar go through with shinigami. The biggest thing is that as discussing with Ichigo, he has under gone a full transformation in a hollow. Why not an arrancar making a full transformation into a shinigami. The reverse must be true as well. It's got to make you wonder what it would be like to see.