I have a Shelley wallet that I have installed for a long time I think it's 3.XX version. Its not updated since so my question, what sort of things are in the new one?
As I got it you speak about one year old Daedalus. If it is so, then your old wallet lacks the most important feature added around three months ago. Newest versions of Daedalus have added support to hardware wallets like Ledger nano and such. Generally speaking, since 4.0.3 wallets created by Daedalus became multi-asset ones because they support native tokens other than ADA itself.
It also generating new addresses to which I tried sending ADA to the newly generated address, I received it and all in all, I got the tokens added to the previous wallet which I have been staking for almost a year. I only need the 15 words(mnemonic) for the two addresses since its just one Shelley wallet I created, is this right?
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Frankly, I don't get this part. Shelley wallet requires 24-words SEED. 15 words(mnemonic) was relevant to Reward wallet. Please, check and make your question clear.
I actually have Yoroi which I have staked my ADA there for some time already. The previous version of Yoroi generates a new address every time you have already used the first address. And since I have sent another batch of ADA to this wallet using the newly generated address, it's been added to the old which I get the total amount of tokens staked.
I was confused why I didn't get the private keys or seeds to this newly generated ADA address. I guess the previous seed of this Shelley wallet applies to it?