Unfortunately I can read all messages so a question: Is "lifetime" always fixed when you upgrade different levels?
Yes, but it doesn't matter:
When upgrading, you lose nothing, not even the "hours of timeline" left on your previous account, because when you upgrade, the timeline is reset but you have a discount on the price of upgrade corresponding to how much you spent already (account by account - so if you spent 3 BTC on three account, you won't have a 3 BTC discount on one account, you will have a 1 BTC discount on each of them).