I wonder if, when using the Master of Blades mark of distinction to switch the stats of currently used wapon to another's (eg. power sword used with characteristic of power fist), the chatacter gains bonuses gained from signature wargear (master) choosen for the sword, or the fact that the sword itself is master-crafted (both beeing +10 to ws tests).

I mean that the char is still using the sword (in using which he is very skilled, MoB beeing even higher level of mastery), and it still is a master-crafted weapon, so I assume that the buffs still apply.

What do you think.

Would be good if you can give us the page and the book where you have found the master of blades

P93-94 Rite of battle... to me its DM choise.

But I will let it be that the 2 weapons are considered mastercraft since:

you are using the sword in a profile of an axe for exemple: you gain an equivalent quality axe (masterwork etc.). It is always the sword that it is used.

Stated that it is a fighting style, not the actual weapon that is changed.

Hope it help. Still, I'm new in this so wait someone better answer

Since the weapon isn't actually mutating into another weapon, I would assume that any bonuses from Signature Wargear or Exceptional/Master Craftsmanship would still apply.

Up to the player's DM's call. First the DM will have to choose Chain, power or Force. If he's not a Librarian the DM will logically choose a category eg chain or power. The player then chooses the two weapons. So if he's using a sword, he can choose the other to be a power fist.

The trickly portion is when the DM chooses another category of weapon; for example, Chain weapons instead of power weapons. Personally I think it'd be illogical if the DM gave him something that is lower than his mastercrafted signature weapon. Any other opinions on this guys?

As far as the OP's question, it seems to me that since you are just changing your 'fighting style', your weapon would retain any bonuses from Craftsmanship and additionally Signature Wargear talents. You're not an Obliterator, transforming your hands into new weapons, and even if you were I would say that if you started with a Master-Crafted Weapon, you should end with a Master-Crafted weapon. After all, you shelled out the Requisition/Exp for that other weapon's craftsmanship. I could see making an exception if the two weapons selected were wildly different in Requisition cost, but to me this is just added bookkeeping.

I actually have a few questions of my own regarding this Deed, though.

1) The Master of Blades Deed allows a player to take the profile of one weapon, and switch it with the profile of another weapon of the same type (Chain, Power, or Force). What, exactly, constitutes a weapon's profile? Damage? Penetration? Weapon Qualities? Special Rules (for example, the bonus damage on Lightning Claws)?

2) What kind of action is switching a weapon's profile? Free, Ready, Half, Full, or something else?

3) Say, for example, I wanted to change my weapon profile to a Lightning Claw at the start of my turn, and to a Power Sword at the end of my turn. Can I change a weapon's profile more than one time in a round?

4) If I have Astartes Weapon Specialization (Lightning Claws), and choose to switch my weapon's profile to that of a Power Sword, do I still gain the benefits of Astartes Weapon Specialization?

5) Assuming that the Lightning Claws still retain their 'Special' quality (it wasn't listed in the Errata, which I think is just a typo), if I change my Lightning Claw's profile to that of a Power Sword, do I still gain the benefits from the Lightning Claw's 'Special' Quality?

1) The Master of Blades Deed allows a player to take the profile of one weapon, and switch it with the profile of another weapon of the same type (Chain, Power, or Force). What, exactly, constitutes a weapon's profile? Damage? Penetration? Weapon Qualities? Special Rules (for example, the bonus damage on Lightning Claws)?

- The normal weapon profile listed in the weapon entry, so yes if you use a power sword as a lightning claw you get the extra damage for degrees of success etc.

2) What kind of action is switching a weapon's profile? Free, Ready, Half, Full, or something else?

- it says "at the beginning of your turn" so I'd say free but you only get to do it the once.

3) Say, for example, I wanted to change my weapon profile to a Lightning Claw at the start of my turn, and to a Power Sword at the end of my turn. Can I change a weapon's profile more than one time in a round?

- as above

4) If I have Astartes Weapon Specialization (Lightning Claws), and choose to switch my weapon's profile to that of a Power Sword, do I still gain the benefits of Astartes Weapon Specialization?

- since you would have Astartes Weapon Specialization: Melee (Power) by the description in the book then it would apply to all melee power weapons so yes.

5) Assuming that the Lightning Claws still retain their 'Special' quality (it wasn't listed in the Errata, which I think is just a typo), if I change my Lightning Claw's profile to that of a Power Sword, do I still gain the benefits from the Lightning Claw's 'Special' Quality?

- Since you use the stat block of the weapon you are using it as then no, you are trading the 'special' qualities of the claws for the balanced qualities of the sword. just like when using a power sword as lightning claw it is no longer balanced.

The fun comes when you wield a power sword in one hand and a lightning claw in the other, so you can actualy use one hand for stuff but get the benefits of using two claws if you want.