Bad Paper said:
The statue cannot walk/fight as normal. Compare to the Iron Body spell, which grants damage reduction. Statue grants hardness, which is something objects have.
Yes, indeed, compare it to the iron body spell. See how explicit iron body is and how statue is not so explicit. If, however, you say that the caster becomes an (unanimated) object, that has a lot more consequences than just hardness and lack of movement. Let me try to list some:
1. Hardness - the most obvious, explicitly listed.
2. Hit points - how many inches thick is the caster? He should now have 15*inches hit points. Or, is a very large object and now has hit points for his arms, legs, torso, etc.
3. Death - an object that reaches 0 hit points is destroyed. No longer can the caster by considered "dying."
4. Energy attacks - half damage from electricity and fire, quarter damage from cold
5. Ranged weapons - half damage from ranged weapons
6. Movement - cannot move, though he will never be considered helpless because he is not a creature
7. Immunities - immune to nonlethal and critical damage
8. Type - since he's no longer a creature, he is no longer affected by anything affecting his creature type; e.g. immune to charm person, charm monster, or anything affecting a creature, living or not
9. Vulnerabilities - based on DM fiat, he could be vulnerable to a miner's pick, taking double damage and ignoring hardness
10. Repaired - Could be repaired by anyone with craft (statue)
11. All saving throws now at 2 + 1/2 caster level.
12. Breaking - could be totally destroyed (i.e. killed) with a break DC what? 30?
I still don't understand why you choose to arbitrarily make it unanimated instead of animated. Anyway, the above list is not exhaustive. I'm sure there are some I'm missing. I think that #8 has the biggest impact. Not being a creature, he doesn't even have any stats. No classes. No caster level. What else does it do?
Bad Paper said:
Does the changing back and forth always happen for free? In one round, can I: change to flesh, act, move, and change to stone?
Returning to "statue state" is a free action, it doesn't say what kind of action returning to its "normal state" is. Should the DM make it a free action, though, you can do it as often as the DM will allow since unless otherwise restricted, the number of free actions is limited only by the DM.