I'd say that you are probably good to "go east young man" but would totally understand preemptively replacing the battery while you are where they will be more readily available.
Yes, they are 3.5 years old and in the bad old days, that alone would have had me replace a flooded lead acid battery. AGM batteries like ours either die in infancy or last a long time if treated well and you are continually using yours and not letting it drain, like so many snow-bound bikes will. So if any will last, I would think yours should.
The only objective way to tell the state of health of your current battery would be to do a draw-down test if you can find a car electric shop in the town you are at. Beyond that is thinking about whether the battery performance is perceptibly changing. One of the best things about lead-acid (including AGM) is that it will generally give a little warning that it is losing capacity if people listen for slower cranking, dimmer lights when at idle, etc.
As a "just in case," please copy the Yuasa catalog from here to your computer:
http://www.yuasabatteries.com/pdfs/Yuasa_Specs_Apps_2014.pdf
The reason for copying it is so that if you are stuck in the hinterlands and wonder what you can borrow a battery from, it has the applicability charts and just about everything else you want to know. The key constraints on the Tenere are the dimensions and the cold cranking amps, shown on page 8. Our OEM battery is the YTZ12S with 210 CCA. Skimming through the other batteries, the YTZ14S is the same case with .7 lbs more lead, giving it 230 CCA. There is nothing else listed which will both fit the tight battery mounting location and give the CCA we need in the big twin.
When it comes time to replace, I will go for the YTZ14S and it's my recommendation to anybody else. While I understand the appeal of lithium batteries and would keep one if the bike already had it, I do not recommend lithium batteries in a big adventure bike where the negatives are not offset by saving a little weight which our bikes carry down low.
So what else uses the YTZ12S or 14S? Fortunately, the BMW 1200GS/GSA from 2009-2013 does and those are pretty common. The earlier GS bikes use the taller YTX which won't physically fit. Some of the Honda twins and VFRs use our battery, and there are a few other bikes. In context, this means that in bigger and more affluent areas you ought to be able to find a replacement. The flip side is that you are leaving the more affluent and populated part of the world and going to where 1200cc bikes become rare.
Since you have the time, I'd suggest trying to find a car shop to do the draw-down test and see if the OEM YTZ12S can get near the 11 amp-hours shown in the Yuasa catalog. But I'd also press on eastward if I did not find such a shop and consider whether performance has perceptibly changed as I passed through bigger cities. Obviously there are no guarantees, but you should be fine.
