Strapping a shield to your arm to gain its shield bonus to your AC, or unstrapping and dropping a shield so you can use your shield hand for another purpose, requires a move action. If you have a base attack bonus of +1 or higher, you can ready or drop a shield as a free action combined with a regular move.
Dropping a carried (but not worn) shield is a free action.
Drawing or sheathing a weapon:
Draw or Sheathe a Weapon
Drawing a weapon so that you can use it in combat, or putting it away so that you have a free hand, requires a move action. This action also applies to weapon-like objects carried in easy reach, such as wands. If your weapon or weapon-like object is stored in a pack or otherwise out of easy reach, treat this action as retrieving a stored item.
If you have a base attack bonus of +1 or higher, you may draw a weapon as a free action combined with a regular move. If you have the Two-Weapon Fighting feat, you can draw two light or one-handed weapons in the time it would normally take you to draw one.
Drawing ammunition for use with a ranged weapon (such as arrows, bolts, sling bullets, or shuriken) is a free action.
If what I am reading is correct, then this chain of actions should be viable.
You are wielding a shield and 1 handed weapon and are 5 feet from your target. On your turn, you decide to take a move action of a 5 foot step to the target, in that move action you unstrap your shield because your BAB is +1 or higher, drop your current weapon and shield as free action, draw your 2-handed weapon because your BAB is +1 or higher, then proceed to full round attack your target because the only true movement you did was a 5 foot step. Is this legal?
A "5-foot step" is not a "move action", regular or otherwise. It's a "miscellaneous action":
Core Rulebook, Combat, Actions wrote:
Drawing a weapon so that you can use it in combat, or putting it away so that you have a free hand, requires a move action.
And:
Core Rulebook, Combat, Actions wrote:
If you have a base attack bonus of +1 or higher, you may draw a weapon as a free action combined with a regular move.
And most importantly:
Core Rulebook, Combat, Actions wrote:
Miscellaneous Actions
The following actions take a variable amount of time to accomplish or otherwise work differently than other actions.
Take 5-Foot Step
You can move 5 feet in any round when you don't perform any other kind of movement. Taking this 5-foot step never provokes an attack of opportunity. You can't take more than one 5-foot step in a round, and you can't take a 5-foot step in the same round that you move any distance.
A "5-foot step" is not a "move action", regular or otherwise. It's a "miscellaneous action":
Core Rulebook, Combat, Actions wrote:
Drawing a weapon so that you can use it in combat, or putting it away so that you have a free hand, requires a move action.
And:
Core Rulebook, Combat, Actions wrote:
If you have a base attack bonus of +1 or higher, you may draw a weapon as a free action combined with a regular move.
And most importantly:
Core Rulebook, Combat, Actions wrote:
Miscellaneous Actions
The following actions take a variable amount of time to accomplish or otherwise work differently than other actions.
Take 5-Foot Step
You can move 5 feet in any round when you don't perform any other kind of movement. Taking this 5-foot step never provokes an attack of opportunity. You can't take more than one 5-foot step in a round, and you can't take a 5-foot step in the same round that you move any distance.
Hmm, I see. It counts as no action, just that it "replaces" a move action. The same question applies then, but the distance is 10 feet away, not 5.
The opposite goes for the example as well. Starting with a 2 handed weapon and moving into a 1 handed/shield combination. Taking away the full attack and replacing it with a standard for both examples.
A 5-foot step doesn't actually "replace a move action". It is its own special kind of miscellaneous action that you can use any time. You can even use it if you use a move action to do something that isn't a "regular move", such as standing up or getting an item out of a belt pouch. So, you could, for example, take a 5-foot step away from an enemy, use a move action to ready a potion, then use your standard action to drink the potion - this sequence uses a 5-foot step, a move action, and a standard action.
Note that "regular move" requires you to actually move. Like, (usually) walk with your feet (but more than a 5-foot step), although flying, swimming, etc., are other ways to make a "regular move". There are other "Move Actions" like standing up, opening a door, etc., but those are not a "regular move".
If you use a move action for anything at all, thn you only have a standard action left (with which you can only make one attack) so you cannot (normally) Full Attack at all if you move.
So, if you have a shield and 1H weapon readied, and you decide to use a Move Action to move ANY distance to approach an opponent, then, as part of that "regular move" you can drop the weapon as a free action and, if you have BAB 1+, you can also drop the shield as a free action. No problem. You can also draw a weapon as a free action as part of that "regular move" if your BAB is 1 or higher, so drawing your 2H weapon is also no problem.
But since you now have only a Standard Action left, you may only attack your enemy one time. Period. So the answer to your edited question about whether you can do this with a standard attack is "Yes", but not with a full attack.
You must use a quickdraw shield (APG) in concert with the quickdraw feat to do this. On the bright side that basically gets you all the benefit of three handed sword and board except on and against AoOs. And against readied actions, of course. In this context quickdraw becomes a defense feat that isn't Crane Style that doesn't suck.
Or now that I think about it you could get an animated shield and save yourself a feat. And have a better shield since you can animate a heavy shield and quickdraw shields are light. Actually, given that the price gap between a +X animated shield and a +X+1 quickdraw shield is half the price gap between a +X keen weapon and a +X weapon the animated property is a pretty cheap price to save a feat.
