Tins oxidise. Invariably. Whether this ruins the tobacco inside or changes it in any way is not something I've personally found out (or are at all interested in sussing out, ever) since I religiously transfer the tobacco to a jar as soon as it arrives. There're only three tins of HH Vintage Syrian in my cellar that I've left unopened, because I wanted them for speculative pursuits. Everything else is jarred.
Tobacco that I jarred three years ago is still fresh and moist in the jars even though I open them every now and then.