When changing the feed do it slowly by increasing the amount of the new feed over a period of 7-10 days.
As the cause of the sneezing is still unknown, you could just brew them some nice tea consisting of oregano, thyme, aniseed, chamomile and fennel. Let it steep for at least 4-5 hours before...
De-worming will not harm them in any way and in doing so they will not be compromised by the damage a worm infestation would cause to their digestive and nervous system.
The new feed does contain corn/maize which can lead to yellowish droppings.
Genetically modified soya can also lead to...