There is more reason to consider a transition from azathioprine to another agent, such as mycophenolate than to take the risk of myelosuppression.Mycophenolate is safe in combination with febuxostat.Sofue et al., PMID 24600205
Generally, I would avoid using both allopurinol (and/or febuxostat) and azathioprine (Imuran) together. The risk of course is bone marrow aplasia, as we know.