I don't see how any DM could reasonably assume the change involves anything else than ***poof*** new form. This isn't werewolf in london here, it is more like sword in the stone. The change happens in a fraction of a 6 second round, is instantaneous, and can include forms as diverse as a spider, lizard, triceratops, or an elemental. There is no slow steady gain or loss of mass it is instantaneous transformation, muscles and bones don't grow or shrink they just change *poof* it's magic. You can choose if your equipment comes with you or not, you can even pick and choose different worn or carried items to do different things. This ring merges with form, this one drops, my hat can stay on my head, my armor will meld with me, the potion bottle in my hand will merge, and so on. I doubt any of that would be possible if it just wasn't a magical instantaneous simple action.