Here's the thread you need to look at. You're looking for Original Routing Credit (ORC).
https://www.flyertalk.com/forum/brit...r-s-guide.html
I suspect you'll receive it without too much trouble. My only concern is that you volunteered for the change but if you can point to the conversation with the BA agent who confirmed you'd receive the points I think it will be ok.
Another option which was extremely fast for me last time (just a couple of weeks ago) is to send a private message to BAMissingAvios.
Let us know how it goes or perhaps post a response in the thread above. The fact that's it's an IB ticket shouldn't matter as it doesn't need to be on a BA ticket number. I routinely receive ORC on AA 001 tickets.