Hi Denise,
The best thing I can recommend is getting a re-check at your veterinarian. With changes in appetite, some medical conditions could be appearing such as hyperthyroidism (even though you haven’t seen weight loss yet). A full blood panel should be done to help rule out some common problems. Secondly, that diet change and hairball issues could be related. Frequent hairballs could be a sign of intestinal issues. And yes, it is possible (though uncommon) for hairballs to accumulate in the stomach in cats. An X-ray (radiograph) could help eliminate that as a possibility but again, in general, getting some tests done would be the safest recommendation.
I hope this helps.
Dr. Ryan Llera