Yes you are correct about that nodes that already have the transaction in their memory pool will not accept the double spend.
You can still transmit a double spend any time you like.
As you say if the old transaction is in all possible nodes then will the new transaction not be accepted... if not rebroadcasterd then will they will eventually they be dropped.
Cheers
ah, okay. I actually just about to try that myself when electrum stopped me because the fee is too low (0.00001
BTC). apparently we can't send low fee transactions as of 2.5.4.
@OP
it seems that are using a version that doesn't give that warning. you should update to the latest version.