Activation Lock - Can't get past this on older phone

I have an old iphone 6S? and my son wants to use it even though it's old (he's 7).

Anyway - the screen says "Activation Lock" This iPhone is linked to an Apple ID. Enter the Apple ID and password that were used to set up this iPhone. {*******@comcast.net}

HOWEVER- after I got this phone I changed my Apple ID to my email address ending @yahoo.com.

If I enter my current info with the yahoo email it doesn't work. If I enter the old comcast email it doesn't work because I don't know what the old password would have been. What do I do now???