Potable antifreeze change after a year?

Antifreeze doesn't 'break down' but as Grant states correctly above, it can and will evaporate.  I would highly doubt it would do so in a closed system as in the H and C water lines but is probable in an open condition as in the sink and shower traps.  One of the houses I winterize in northern Michigan will get the RV antifreeze in toilets and all traps this month.  By the time I open things up come May it is easy to see how much as actually evaporated in the toilet bowls, usually 1/2" over 6 to 7 months.  Your call geewizard, I would pass on the water lines but may throw some in the traps.  Even if you didn't, the worst that could happen would be a "smell" from your gray or black water tank if all did indeed evaporate from the trap. 

I use compressed air  and after that quick process is complete I measure out a total of 16 oz. RV antifreeze for 1 shower and 2 sink traps and use a little bit for the top of the toilet seal to keep the seal moist during the winter months.  The last time I purchased a gallon RV antifreeze was 2012 and the price marked was 79 cents - 1 gallon will last 8 years.  Curious to see what it sells for now, will probably purchase a gallon later this week.