I would leave it until next feeding time they may be more hungry then, give them a mix of old and a small bit of new kibble and a litle tripe on top as a taster.
The next meal, mix in a little more of the new kibble and put a little tripe on top as a taster.
Oakley will eat kibble plain but Merlin has to have something - even if its only a teaspoon of something mixed in.
Can I ask if your husband has had dogs before? Is it you that is cleaning up the sick?