I have an iPhone with an eSim and physical SIM and therefore 2 different phone numbers.
There is a message thread in Messages that contains messages sent to both of these numbers.
I want to know, when replying to the last message, how do I choose the phone number I am sending from?
The Apple documentation explains how to do this when starting a new message thread for a contact with no chat history:
You can use iMessage or SMS/MMS to send messages with either phone number.* You can switch phone numbers before you send an iMessage or SMS/MMS message. Here’s how:
Open Messages. Tap the New button, in the top right-hand corner of the screen. Enter your contact's name. Tap the current phone number. Tap the number that you want to use.
However, it is conspicuously quiet about my query. I want to choose the number for sending to a contact with an existing chat history, without deleting prior messages and/or removing SIM phone numbers.