Tobacco Ageing Query.

G.L. Please has some interesting articles on aging. I'll see if I can find and link to them.
The first thing i thought was that the popped tin will probably dry out eventually.

http://www.glpease.com/FAQ.html#AGE

Let's assume for the sake of argument that you reseal the one that you opened so that it doesn't dry out. Which one would do better over time, the one that had been opened, or the one that has not been opened.
The answer is...they will be slightly different. And, you may not detect the difference, or you might prefer the one that got opened over the one that didn't, or vice versa. There's no correct answer except that the two tins will be slightly different.

I have found 12 months make a large difference in C&D and GLP. Not so much with Mcclelands.

There are two types of bacteria that are part of the aging process, aerobic and anaerobic. Aerobic bacteria need oxygen to do their job while anaerobic bacteria don't become active until there is a lack of oxygen. Both affect a blend in different ways during aging. So, yes, you do need some oxygen at the beginning of the aging process so that the aerobic bacteria can perform their part of the fermentation process. Once that oxygen is depleted then the anaerobic bacteria continue the fermentation process and the aerobic bacteria go dormant or die. IMO, a well aged blend has benefited from the influence of BOTH interactions.
That isn't to say that you can't age tobacco completely aerobically, the cigar guys do it all the time. They constantly introduce fresh oxygen into the process. The result isn't bad (I love a well aged cigar) but as others have said, it will be different.
In your scenario, what you have done is "reboot" the process. By cracking a previously sealed tin, any anaerobic bacteria go dormant and any aerobic bacteria reactive and the process starts all over again, provided the tin is resealed to an airtight state and provided the tin had been sealed long enough for the anaerobic process to begin.
That is, of course, a very simplified explanation of the aging process but should give you an idea of the different interactions happening inside a sealed tin of tobacco.

There are two types of bacteria that are part of the aging process, aerobic and anaerobic

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This is assuming tobacco actually "ferments" as it ages. To date, there is no definitive answers to how tobacco actually ages. Most research in trade journals tend towards a chemical reaction.

I'm one that has rarely noticed a difference and/or improvement in the tobaccos I have aged and I have yet to come by a blend that noticeably tastes better than when I first purchased it. I have noticed a huge decline in blends containing Latakia and once a Lat blend gets over the 5 year mark, its as good as garbage to me. I don't like the way they age. Don't get me wrong, I cellar a lot of tobacco and a part of me still hopes to find an amazing aged blend but I'm not holding my breath. As of right now I prefer my tobacco relatively fresh. I hoard a lot of tobacco and I have been since 2009 but I don't really do it with "aging" in mind rather I'm afraid I'll never be able to get said blend again and I want enough to last me a lifetime. So far I haven't noticed a decline in Va and Bur based blends and that's all that matters to me.

I hoard a lot of tobacco and I have been since 2009 but I don't really do it with "aging" in mind rather I'm afraid I'll never be able to get said blend again and I want enough to last me a lifetime. So far I haven't noticed a decline in Va and Bur based blends and that's all that matters to me.

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You shouldn't notice a decline with Va/Bur, Va/ Va/Per blends for many many years, decades in fact. Burleys don't change a whole lot and Virginias can get sweeter and/or tangier and deeper for decades.
A lot of people like the taste of faded softened Latakia. They find it more mellow. So there's something for everybody. But Latakia should still retain its punch after 5 years. After 10-15 you should see some lessening of its strength, and after 30 years you probably will barely taste it was even there. Personally, rather than spending the farm on famous faded Latakia blends, I'll just buy lighter blends that I like, or mix something up.