(Idea sketch) defensive/offensive combat maneuvers

Some idea I had for combat rules for a given game.

Armor numbers being something like: 

Light armor: 3
Better armor. 4
Best armor: 5

Then you have a panoplia of weapons with one assigned dice each, for example:

Greatsword: d10
Sword:d8
Dagger:d6

etc.

If you get your weapon and hit greater than the armor, you deal a hit (scratch a heart away, roll for damage or whatever you like to do afterwards)
But after you have rolled "to hit", you can choose to convert that result in your armor score this turn, then roll for attack again.
This can be used for both defensive and offensive maneuvers, and represent the parrying potential of certain weapons, while also adding some "choice" into combat.

Example:

Your PC, armed with Light Armor and a Sword, fights an enemy who has Best armor and a Greatsword.
You know that the greatsword will easily hit an armor of 3 with a d10
You attack first, and roll the sword's d8, and get a 6 (which is over the enemy's armor score)
You can choose between dealing a hit OR adopt a parrying style, converting the 6 into your armor score for the round and trying your luck rolling "to hit" again.

The same idea can be used aggresivelly: Lets say that you rolled a 1 instead. You can choose to keep that first roll as your armor (lowering your armor to a score of 1) but you would get to roll again and maybe take your enemy's last hit.

Certain weapons can be worked out so they have different damage dice on the first roll than in the second, to illustrate they are more defensive or offensive in nature. For example, a spear can have a d10 on the first attack, capable of defending up to 10 ac, but have a secondary attack of d6.
Great axes can be done in reverse: a first attack of 1d6 and a second of 1d12. The first one will probably get you exposed in order to deal the big, bad second attack.

As always, unarmed combat is difficult to model, as anything worse than a d6 will never go through best armor unless the enemy chooses to adopt a worse ac for some reason. One possible way to do it is to have unarmed combat go like (First attack d4/Second attack d6)

Shields can be done by adding an extra d6 to the first attack roll, choosing the best of the two. In this way, it provides help both in the defensive mode and the offensive mode. The shield die cannot be used to deal damage, only to be put as armor... unless the master decides otherwise. I must make the numbers so the combination of sword and shield doesnt deal both better defense and attack than a greatsword, which would render it useless.