Pre-Covid, they typically only offerred refundable fares in the higher fare classes such M, H, Q, or K and you would often have to upfare to a higher fare class to purchase one. This would result in a higher upgrade priority than the lower fares classes which were all non-refundable. Post-Covid, they have refundable fare filings going all the way to down to V fares (typically starting at around a $30 upcharge from the non-refundable Main equivalent fare). Since you will have the same fare class in both cases, there's no upgrade priority benefit.

Here's an example from DTW-ORD with cheapest Main fare which has a refundable variant for $30 more. They are V fares in both cases, so there is no upgrade priority benefit.