Chaitanya, the listed number of ohms for a speaker represents how much the wire in the speaker drivers resists the flow of power through them. The lower the ohms, the lower the resistance(or impedance, which is the more specific term), and more current can flow through the wires for a given amount of voltage "pushing" it through. Amplifiers, including those in receivers, don't know what the impedance of speakers they're connected to is, just that they have to supply a certain amount of voltage and current at a given instant in time. So yes, the receiver can be said to "automatically" adjust to the impedance of the speakers as long as it isn't called upon to supply more voltage and current than it's been designed for.
The lower impedance switch or menu setting that many receivers have just limits the maximum amount of voltage that the receiver can supply, and when the maximum voltage is limited so is the maximum current and power, because of Ohm's Law. This is a safety-related measure to help prevent overheating, but because it reduces the maximum power available by as much as half, it should never be used.
Your 325 will drive your Axioms very well and the 50 watt rating is quite conservative, so the sound level can be quite high. If you want to change to a new receiver because of HDMI or other features, that's a topic that can be considered later.
