Good news is, the concentrates will keep much better than the mixed developer will. Given they're near 25 years old, however, I'd probably test first with a roll shot specifically for this.
The beauty of liquid concentrates is that you can mix a partial batch, and as long as you measure accurately and do the arithmetic correctly, all will be well. That means you can mix one liter of the 10L package, use that for 6-8 rolls over a few months, and when it's used up or goes off, and you're ready to develop more, mix another liter.
My recommendation for bottles is to buy store brand club soda in 1L size. This comes in PET that is very impermeable and has a gas tight cap (bottle and cap are good for at least 120 PSI carbonation pressure, and will hold that pressure for many months on the store shelves or in your pantry). Dump the contents, rinse the bottle, remove the label and relabel with masking tape and a Sharpie, and when you pour the developer etc. into the bottle, squeeze it a bit to eliminate air before closing the lid tightly. Far better than accordion bottles, far cheaper (I paid 79 cents each for mine), and they're clear, so you can see if the solution has changed color or dropped a precipitate. Also pretty easy to clean due to their shape, but cheap enough to just recycle (rinse first, please) -- they'll get shredded, cleaned, and made into some non-food-grade plastic items (or look on YouTube to make them into rope, yourself). Don't try to pour from these bottles into your developing tank, though; the small necks make it impossible to get a rapid fill. Get graduated pitchers larger than your biggest tank for that.
I've got a pair of Diafine bottles (PET beverage bottles) that I mixed in 2005 and had in a shed (temperatures from near 0F to above 100F over the course of a year) for five years -- contents are still the same color they were when I last used them.
For the concentrates, it surely would do no harm to gas blanket the liquid in each bottle before resealing them. For this, you use an inert (to photo chemicals) gas that's heavier than air. I've used butane lighter fuel in the past, with good results, but for about twice the cost you can buy commercially packaged argon. Both work equally well, but argon is slightly safer (if you don't smoke in your darkroom, there's almost no difference).
Thanks for the advice
