To report: a week later I find that three out of the four iDines have posted without any intervention from me. Only the earliest one didn't. So, it would appear that, as long as you change the number before IDine have time to process the awards, you're OK. I had even pre-booked one that posted using the "old" MasterCard number, but it still went through.

I have now written to them sending the receipt for the earliest iDine and await their response.