Forget Japan unless you can pass an astronaut medical and fancy spending 12-18 months doing your whole licence all over again.
Japan will require at least a year, all the written exams, flight tests and the physical standards of an astronaut before he starts in the right hand seat again with who knows how long to upgrade
Okay, let's remove some of the exaggeration so the OP can make an informed decision.
1. The medical is not to astronaut standards. They require you to have a reasonable BMI (with, I believe, a limit of 28 for the initial medical) and the tests for the initial are no different to any other initial test I've had under EASA, FAA & CASA. I will admit they do seem to be keen on eye tests but the field-of-view test is the only one I'd never come across before. ECG & chest X-ray are normal. Stress ECG (treadmill) on the initial but that's not a big deal. EEG on the initial and then not repeated. Blood tests are an annual thing which seems to be an Asian thing, because the 'Western' medicals only seem to have the pin prick for haemoglobin. Hearing test is standard. Now, I know about a lot of the medicals in the US etc where walking into the clinic counts as a pass but the Japanese ones are not the intensive and difficult tests that rumour has them to be. A rumour probably touted by those with a BMI of 40 who were out on the town the night before and wondered why they weren't successful.
2. You won't be under training for 12-18 months. Yes, the training is far too long compared to how it's done almost everywhere else but even direct entry captains, with a longer course than FOs, will get it done in 7 months. I believe AJX have shortened this because direct entry captains are now required to have a JCAB ATPL, so obviously they don't have to do a lot of the initial stuff.
3. You won't do your whole licence all over again (especially when you consider the EASA exams). You'll do air law and radio written tests and those, combined with the sim check, are enough for them to convert your foreign ATPL to a JCAB version. No difference to anywhere else in the world really; well, I've always had to do an air law exam to convert my licences. This, under the AJX scheme, is done when you first get to Japan and then they crack on with the 767 ground school etc. It only takes a couple of weeks.
4. Upgrades really are not a matter of 'who knows'. Our guys get assessed around the 3 year mark and then start their upgrade 'training' at about 3.5 years. Of course, they have to be able to demonstrate the unique qualities that are required by the Japanese (knowledge of irrelevant rules being far more important than flying ability or management skills) but with 3 years under your belt you'll be more than equipped to deal with that BS.
Japan is not my favourite Asian country and I could have a LOT more fun in Vietnam, Thailand etc BUT I do not live in Japan; it is ONLY a place of work. I go home when I've done my stint and that is far, far more important than where I can get the cheapest beer. Places like Myanmar are great places to visit but I would not want to work there. Still very 3rd world and with wages to match their economy. It kind of reminds me of backpacker hotels; they're a bit of fun when you're young and things don't matter too much but then you get older and more serious and 5* become more your thing. Not as much fun but more comfortable and a lot more stable.