I've never heard of Modesto Millings so I looked it up, all I could find is the Modesto Milling Organic Non-GMO Rabbit Food, if that is what you mean it does not look like a good food for chins at all, it has several ingredients that aren't good for chins. Currently the foods most advised/recommended foods are Oxbow Essentials and Mazuri. Aside from that though, no, I would not advise giving pellets only. Chew toys are good for wearing down front teeth but hay is required to properly wear down the molars, ideally feed a variety of hay, with timothy as the primary hay. Each kind of hay is chewed differently so it can help more evenly wear down the molars as they chew it up. Hay is also important for digestive health, the extra fiber in their diet helps keep them pooping.
There are two ways to switch over pellets, one is to slowly switch over a few weeks. For example, first week do 75% Modesto 25% Oxbow, then the next week 50/50, next week 25/75, then the forth week 100% oxbow. The other way, if you don't have enough to do a few week switch or the chin is picking out just one pellet type when you try to switch, then you can just take the pellets away completely for a few days and feed hay only, then switch directly over to the oxbow, that gives the gut time to bulk up on fiber to hopefully avoid stomach upset.
Just to be clear, only the Oxbow Essentials is good, they have come out with a couple other chin foods (Simple Harvest and Garden Select) recently that aren't very good.