The change in rate (aside from seasonal temperature fluctuations) may have been due to either crystal ageing or drop in battery power. Each of these things has been known to have an effect on rate and of the two I believe a dieing battery is by far the more commonly reported problem.
If you change the battery yourself, you might see the rate return to normal.
On the other hand, if you send it back to Japan the watch will be gone for several months at least and from my personal experience I have absolutely no faith in Citizen's ability to regulate the rates of their top tier watches.
If you change the battery yourself, you might see the rate return to normal.
On the other hand, if you send it back to Japan the watch will be gone for several months at least and from my personal experience I have absolutely no faith in Citizen's ability to regulate the rates of their top tier watches.