After you book, you can continually check the reservation in "My Trips" and look to see if cash/mileage upgrades are being offered. The offers are dynamic and may go up or down or disappear completely (and then reappear later). The offer may be better than refaring or it may be so ridiculously expensive that it'd be cheapre to just refare and buy D1 outright.

Do you have a DL AmEx and the option of "Pay with Miles"? If so, you could potentially look at using Pay With Miles to reduce the cash cost of the D1 ticket down to what Main/C+/PS would be.

That said, depending on when you're traveling, there are pros and cons to UA versus DL. On DL you would have the nonstop and the flight is marketed as Delta One so you would get SkyClub lounge access at DTW but if you're flying at a time of year when DTW-HNL is being operated with a 767-300, the D1 seat here is often regarded as the worst hard product among DL's D1 offerings while if you go with UA, you'd be on the 787-10 with the Polaris equipped seat. The downside to UA is UA does not market the route as Polaris so you would not get lounge access on the layover.