You can use a 3 wire feeder to supply a separate building, if...
- The installation was in compliance with a previous edition of National Electrical Code (existing premises wiring).
- An equipment grounding conductor is not run with the supply to the structure.
- There are no continuous metallic paths bonded to the grounding system in each structure (bonded water, or gas piping, other conduit, etc.).
- Ground-fault protection of equipment has not been installed on the supply side of the feeders.
National Electrical Code 2014
Article 250 Grounding and Bonding
II. System Grounding
250.32 Buildings or Structures Supplied by a Feeder(s)
or Branch Circuit(s).
(B) Grounded Systems.
(1) Supplied by a Feeder or Branch Circuit. An equipment
grounding conductor as described in 250.118 shall be
run with the supply conductors and be connected to the
building or structure disconnecting means and to the
grounding electrode(s). The equipment grounding conductor
shall be used for grounding or bonding of equipment,
structures, or frames required to be grounded or bonded.
The equipment grounding conductor shall be sized in accordance
with 250.122. Any installed grounded conductor
shall not be connected to the equipment grounding conductor
or to the grounding electrode(s).
Exception No 1: For installations made in compliance
with previous editions of this Code that permitted such connection,
the grounded conductor run with the supply to the
building or structure shall be permitted to serve as the
ground-fault return path if all of the following requirements
continue to be met:
(1) An equipment grounding conductor is not run with the
supply to the building or structure.
(2) There are no continuous metallic paths bonded to the
grounding system in each building or structure involved.
(3) Ground-fault protection of equipment has not been installed
on the supply side of the feeder(s).
If the grounded conductor is used for grounding in accordance
with the provision of this exception, the size of the
grounded conductor shall not be smaller than the larger of
either of the following:
(1) That required by 220.61
(2) That required by 250.122
Changing from a 30A breaker to a 50A breaker can only be done, if you also change the wires to 6 AWG. In which case you'll have to follow current codes, and install 6/3 with ground. Breakers (and fuses) are always sized to protect the wire connected to them, so you can't change the breaker size without also changing the wire size (unless you're going down e.g. 50A to 30A).
However, depending on what you're doing, you may not have to change the breaker at all. If the planned circuits in the structure are not going to be fully loaded, you may well be able to supply the subpanel with a 30A breaker. Just because the subpanel has 50 amperes worth of overcurrent protection, does not mean the supply breaker has to be 50A. Whether or not you actually need a 50A breaker on the supply, depends entirely on what the subpanel will be powering.
There's no longer any difference, aside from the fact that a detached structure needs to be bonded to a separate grounding electrode. In both situations you're now required to run 4 wire cable, and keep the grounded and grounding conductors separate in the second panel.
A detached structure has to be bonded to a grounding electrode, to reduce voltage transients between the structures. This is because the ground "earth" can be at slightly different electrical potentials, even over small distances. Bonding the electrode to the grounding system of the main structure, insures the entire grounding system is at the same potential.
Best Answer
To make this easier, consider the grounding conductor (the ground wire) as a backup neutral. It's not used unless a ground fault occurs, in which case it's only purpose is to provide a path back to the source so that a protective device will trip. (breaker, fuse, etc.)
Your grounding electrode system on the other hand, the ground rod here, is only used for over-voltages on the line such as static or potentially lightning. Although we bond the grounding system, they serve two different purposes.
So for your first question: no, it is not against code to install a ground rod at the subpanel. It is actually required by code.
The only exception to that being that if you were only running a single circuit (or multiwire branch circuit) with a ground wire, then you would not need an electrode system.
Now on to your next question on if a metal stake/metal building works as an electrode: yes and no. There are a few conditions were these could apply, but they do have to have direct contact to the ground (the earth itself, the dirt or concrete below). Check out 250.52 (NEC 2014) for the list of electrodes that can be used.
Lastly, all new feeders to outbuildings must contain a ground wire. Previously they were not required, but this is no longer legal or up to code for new installations. However, modifications to existing 3-wire systems do not require the pulling of a new ground wire as long as it fits a few requirements for the exception; as seen in 250.32(B) (NEC 2014)