Changing Weapons in Combat
When changing weapons in combat, the total number of blows is the lower of the two. So if a dagger with six blows is changed for an axe with four, the character would be treated as having four blows for the entire turn. If more than four have been spent, the character is out of blows until the next turn.
Changing weapons costs one blow. If the old weapon is simply dropped, it costs one blow, but the character can still make an attack (costing an additional blow) at the same time. Sheathing or otherwise putting the weapon away does not allow for an attack on the same turn blow.
The exception to the above is a crossbow (light or heavy). If the crossbow is loaded, it may be fired at a cost of one blow, dropped and another weapon drawn at the cost of one blow, but no additional attack can be made the same turn blow.