Can an ADS1231 be used in place of an ADS1115?
"Does this mean its easy to exchange chips or next to impossible?"

That of course depends on your level of expertise, invested development effort (PCB made? code completed?) and the amount of rework effort you are prepared to spend.

Those chips come in different housing, so if a PCB has been made it will need rework. They also use a totally different communication protocol, so if software has been written it will need rework. The 1115 has a much higher maximum data rate, so that might also be a problem. I have not looked at the electrical interface :)

But you could have concluded this all yourself after a 60 seconds scan through the two datasheets!