There is no such thing as "sub accounts" on the blockchain.
Supposing that by "sub account" you mean you have an account named "top" (which is your LTM account), and using that you have created "top.sub1", "top.sub2" etc.. These are all independent accounts on the blockchain.
The only connection between those is that the sub-accounts have the top accounts as their lifetime referrer, which means the "top" account receives the cashback for the fees paid by the sub accounts. That connection is immutable, AFAIK, the only exception being that it is removed when the sub accounts upgrade to LTM.
Regarding seeds, you can move some of accounts to a new wallet and generate new keys for the accounts in the new wallet.