Uh... you might want to call United to merge all your accounts?
Are you sure you don’t want to try to keep your miles alive? I suggested buying a small denomination eGC using MPX, which will do the trick.
As for GC>Disney GC, your best bet is Walmart. Walmart still allows GC>GC, and if you have/have access to a Sam’s Club membership, you can use a Walmart GC at Sam’s Club or samsclub .com to buy discounted (~4% off) Disney GCs.
Staples, Best Buy, Kohl’s, and TJ Maxx are no go on GC>GC. I don’t think Kohl’s or TJ Maxx even sell Disney GCs. BB&B’s system allows it, but YMMV whether your cashier will let you use a GC to buy a Disney GC, or accuse you of a federal crime.