Frequent-flier miles are both a boon (to the vacationer who scores. Elite members can roll over Medallion or Elite Qualification Miles into the next year. Diamond, Platinum and Gold Medallion members can expect several new benefits on select Alliance. A look at the value of Delta SkyMiles elite status in 2018 based on the benefits offered to Silver, Gold, Platinum and Diamond Medallion members. Next year, Delta will switch its…