I am remodeling my inlaws' house and their shower faucet is Delta and has 2 handles ( one for hot/one for cold). Can I replace the handles with other brands such as Kohler? I was told that the valves in the wall are not universal.

Why do you want to replace the Delta handles with other brand?

I am afraid it is not going to be easy. Every brand has stems of certain configuration that - in most cases - are not inter-changable. However, there are 3rd party manufacturers of conversion kits that will allow that. From my experience, they don't hold well.

These days, Delta makes quite a many designers type handles and trim. Get Delta product catalog. Select trim you like. Then, Call Delta support line. They'll tell you exactly what you need to fit handles of your choice onto your existing faucet.