I have Monero GUI and some XMR in this wallet, but suppose I want to access this wallet in MyMonero (without creating a new mymonero wallet and sending xmr to it), how do I do it? MyMonero requires a 13 word seed, whereas Monera GUI requires 25 word seed.
Even though MyMonero generates a 13 word mnemonic seed by default, it will accept a 25 word mnemonic seed (generated by either the CLI or the GUI) as well. There are basically two options:
You import the 25 word mnemonic seed generated by the CLI/GUI before any transactions are sent to the corresponding address. In this case you won't have to pay an import fee.
You import the 25 word mnemonic seed generated by the CLI/GUI after any transactions are sent to the corresponding address. In this case you will have to pay an import fee (of 0.1 XMR), because MyMonero has to scan the blockchain looking for transactions that belong to your wallet / address, which can be quite resource intensive.