My original YUASA battery is now 6 years old and has given excellent service. Given its age, I decided it was time to replace it. My concern was reduced reliability/performance in sub-zero temps up north.

The new battery (GYZ32HL 500cca) is an upgrade from the original (YIX30L-BS 400cca).

Well, I should have followed my own often given advice of load testing a battery BEFORE replacing it.  It turns out that the 6 year old battery is still providing 480cca. The battery guy was impressed.

So I have a "spare" battery and am wondering what I should do with it. I'm thinking adding it to the Z; placing it under the seat, but other than the extra capacity for the winch, I can't seem to justify the effort.

I thought I'd ask the forum:  What would you do?

[2015 Z Force 800 53/Trail EPS]

I kinda had the same situation on my first battery. It went..what I thought was bad after 4 years. Bought a replacement and install it. Then pulled the top off the OE and saw it was dry so filled it with distilled water and put it on the .75 amp tender for over a week. Thought it was gone but it did finally switch to the green light. Load tested it and all was good. Thought I would put it in as an extra battery but had no real use for it...no plow..no winching usually..no extras so for me it was just extra weight. I ended up after a few months of it sitting on the bench just tossing it out.

Yours at 6 years is well past it's due-date and there isn't that much time left. Not sure it would be worth it. Sulfation has to be almost to the pivot point.     

I agree, that the 6 year old battery is not likely to give full or adequate service for very long despite it finally reaching green light status on the maintainer.
For its utility as a traveling spare, yes something is better than nothing when you are caught out but I would retire the old one and get and carry one of those small but powerful lithium jumpers. There's almost every advantage: Super light weight and small package, retains its charge for a very long time, useable for recharging cell phones etc, often has a flashlight built in, and you can use it to boost a buddy's ATV without risking your own ECU like you do when jumping him from your installed principal battery.
I'm loathe to discard something that still seems 'good' but in this case I think it more prudent to salute the old battery for being such a survivor, and I'd let it have the well deserved rest.