Honestly @MisterMist I’m sure they’ll be fine. A few days to adjust and they’ll happily eat it (that is if they’re just fussy and there aren’t other issues such as ASD which of course makes it more complex/difficult).
Children are quite adaptable - if they’re given the choice of new ketchup or no ketchup they’ll soon try it and decide that it’s absolutely fine.
And speaking more generally, Heinz isn’t the best or nicest ketchup by any stretch - it’s just the most well known brand.