Oh, I don't know man. Already mentioned what I thought ... and some more.
December 5th, post 293, ya said it was 1ppm. Now, 25 days later, you gained .5ppm, and I am guessing you probably still have 3 more months of Winter ahead of ya to endure. If your readings are correct, then I wouldn't be suprised if the ammonia reaches 4~5ppm if the weather does not warm up around April.
Although, I am a bit more paranoid than how I come across; so, since ya haven't done any water changes, I would probably get serious about doing a trickle flow through and get some ammonia binder, such as
Seachem AmGuard, in case it goes higher. The binder chemical might be the cheapest approach until things warm up so you can install a proper flow through system that can work even during your Winters. Under normal warmer conditions, small water changes would not be very effective to dilute ammonia due to the occurance of a pollutant equilibrium; however, under colder conditions, small water changes might be effective. The binder chemical is definitely the most effective, but it could become expensive depending on the size of your pond and length of your Winter.
I can't remember the exact specifics, but particular test kits will falsely register the binder chemical as toxic ammonia so make sure ya have the correct test kit. There is bacteria that will eventually break down the binder chemical releasing toxic ammonia it bound so you will have to maintain a dosing schedule to keep the ammonia safe until the proper bacteria colonies regain their foothold.
As tanks are cycling, I have read many testimonials of their koi surviving high ammonia levels, but you might never know the affects if they actually do survive it. "Surviving" can be a very relative term.