Hi Henge,

Your Barrett's shouldn't change. It's an addition of cells to protect against acid reflux. It is a permanent addition that doesn't grow or shrink bu remains inert, though there is the small possibility of mutation which can lead to cancer which is why we have regular endoscopies every few years.

Your diet won't make any difference. Acidic foods do not cause the stomach to become more acidic. Acid production is controlled and regulated by hormones and neurotransmitters as described here.
The refluxing acid attacking the oesophagus that triggered the production of Barrett's cells usually inflames the mucosa (oesophagitis) which may be irritated by acid foods passing over it as heartburn. By following the diet, you are just avoiding anything that may cause an unpleasant symptom rather than cure it. Your omeprazole is the best way to do that.

Diarrhoea and constipation are possible side effects from PPIs (Proton Pump Inhibitors - omeprazole is one.) You may find you don't experience those symptoms on a different one. You can ask your doctor if you can try lansoprazole or pantoprazole instead. (If you are on 20mg omeprazole, it's equivalent to 30mg lansoprazole or 40mg pantoprazole.)
PPIs work by preventing some of the parietal cells that line the stomach from making acid.
The alternative way of reducing stomach acid is by H2 blockers which block the H2 receptors that receive histamine signals to make acid. You would normally be prescribed 300mg ranitine or 400mg cimetidine. H2 blockers are not as effective as PPIs.