Each League

For every new league that starts, Legacy being the next one, we will have to roll a fresh character?

That's correct. New character, empty stash. Everything from before will be in standard league.

Thanks.

What about if I bought stash customizations on Breach, do they overwrite my standard stash when league ends?

I beleive if you now buy regular->premium tab upgrade (is this what you mean by customisations?) on Breach you will have one of your regular tabs on Standard upgraded immediately. And when Breach ends your Breach tabs will just became remove-only tabs on Standard, no overwriting.

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I beleive if you now buy regular->premium tab upgrade on Breach you will have one of your regular tabs on Standard upgraded immediately. And when Breach ends your Breach tabs will just became remove-only tabs on Standard, no overwriting.

yeah but you will also have a new premium stash tab in legacy league when it starts and another premium tab in standard (and hardcore/breach hc for that matter).

I believe all the tabs in stash stay the same?
So if you now buy them you still have them in new league?