Byakuya is the only Shinigami as of yet to have multible bankai forms.
He has 3: Senbonzakura Kageyoshi (millions of Cherry blossons, basically)
The black dome with 1000 swords
The 1 sword with a back-halo and wings
I came up with my own rules for making multi-form bankais for my bleach characters. Basically, they can change into anything I want as long as only the shape changes, but not the properties of the bankai change, and in changing the bankais shape, you effect its attibutes. Now this may be confusing, so I will give some examples.
EX1: When Byakuya creates the dome, he doesn't actually change the abilities of his bankai, all he does is rearange it into the shape of 1000 swords, and this in turn changes the bankai's attibutes by compressing it to increase power. (basically, Byakuya compresses his bankai like Ichigo's, but far less extreme.)
Now, byakuya is the only Shinigami to have more then 1 stage to his Bankai thus far, so I will need to dig a bit for my examples if you don't mind >_>
EX2: If Komamura were to develop a second stage for his Bankai, his Armour might compress and form around him, basically Allowing him to use is defensively and increasing its power by compresing it. It still would work the same, but because its shape is different, it can now be used for different situations, the only catch is it need to give up range to do this.
EX3: One of my own characters Bankai basically gives him 1001 blades from his shikai, except he can control them in the air in a Senbon-esque manner. What his shikai does, is basically it can chage its size and shape into anything the Shinigami wants. (yes I know its overpowered, that is not the point) For his second stage-Bankai, he brings all of these blades together and basically uses them to copy someone elses Bankai. He does this by changing his Bankai's shape to mimic theirs, then using the remaining blades emulate the abilities of the bankai. (ie. For Hitsu's Bankai, blades act like ice. For Ichigo, they carry me to increase my speed and rapidly de-compress to mimic Getsuga Tenshou. For Byakuya...well actually thats pretty obviousy >_> At any rate, you get the point. They change shape to mimic the Bankai as closely as possibel, then the broken abilities of the Zanpakuto do the rest.)
*GASP* Finally, until someone else gets another form of bankai, this can not be proven or disproved, but I feel it is a good template and
1: In theory if a Shinigami changes drastically enough, their Zanpakuto would also change.
2: Ichigo has obviously not yet mastered his Bankai, hell, Hitsugaya hasn't mastered his yet, so the degree of control Ichigo has now is bloody impressive.