I understand the following to be technically correct:
1) The simple resistance of copper conductors increases as its temperature increases (not referring to the complex impedance of the speaker to the signal). From this, as the voice coils in your rear speakers heat up, their simple resistance will go up, causing the power dissipation of the channel of the amplifier attached to go down. If power dissipation (watts) goes down, 2 things happen in the amp: a) heat dissipation in the power stage of the respective channel goes down, and b) b/c less power is being dissipated, the effective load on the amp is less, there is less of a total voltage drop across the respective channel, leaving more voltage for the rest of the amplifier to use. For these reasons, I think your amplifier channel will be just fine.
2) If the moving mass of the speaker is being thrown to its maximum +/- displacement due to excessive voltage and being kept from moving further by the surround holding it in place in both extremes, the following would happen: since speaker movement is resisted at the extremes of cone travel, the magnetic field of the current flowing through the voice coil will be resisted greatly by the permanent magnetic field of the magnets in the speaker. Because of this greatly increased physical resistance to movement between the two magnetic fields, the resistance of the voice coil will go up when it is at its +/- extreme physical travel. For the same reasons as item (1), this should not harm the amplifer, as power dissipation will go down.
In summary, I think you amp will be fine, as long as you dont have the gain for the channels that are distorting turned up past where it should be. I have heard through the grapevine that too little power kills speakers before too much power would. Worst case scenario is that you will blow your factory rear speakers. Then you would be in a great position to replace them with some more capable speakers!
Hope this helped, good luck on your audio system. -Brian