I have a 2647 Thinkpad. Should ram from a 2648 also work in my computer.
Welcome to the forum!
Your T23 uses PC133 144-pin laptop DIMMs. As long as the other machine is a T23 as well (not a T20, T21 ot T22) it will work fine.
I have a T23 2647, and have been offered a new ram, stated to be suitable for both a T23 2647 or a T23 2648. My query is, are these interchangeable.
Yes they are.
All T23s use the same type of RAM.