old memory

Obviously you still face compatibility issues with particular models and brands of RAM, hence the RAM compatibility chart on product pages. Generally speaking if a particular kit of RAM worked in an older model board of the same brand, in this instance ASRock, it will work in a newer one without issue so long as the jump is not too great, eg. a socket 775 board to a Z97 as then you may find the much older RAM lacks features needed by the much newer chipset. Even in this instance however the odds are still in favor of the RAM working.

In all honesty, unless you are planning to seriously upgrade the capacity of your RAM the DDR3 1600 kit will be more than adequate for the Z97 board, you generally can overclock most DDR3 significantly if you bump up the timings and increase the voltage a little. Take a look at the specs of say a DDR3 2400 kit from the same series as your vengeance kit and set your 1600 kit to the same in your BIOS and most likely you will find the RAM will quite happily run at DDR3 2400 or beyond. Most DDR3 over 1600mhz uses 1.6v or 1.65v and has fairly loose timings with only the true "enthusiast" RAM actually being much different than a 1600 kit in construction and component quality. Basically it comes down to binning with some kits only being stable at XXXXmhz while others able to perform higher. We the consumer then pay a premium for the higher binned RAM when in all likelihood the cheaper binned RAM would have overclocked to the same speed anyway now that the manufacturing process is so mature.