Currently UALs self certification of prior time is optional. There would have to be massive changes to UALs certification process to make it mandatory, where UAL flight ops assumes the liability/responsibility to track down certified hours breakdowns from pilots previous employers (DoD, previous airlines etc) to meet the 1000hrs SIC requirement. I honestly don’t see that happening before they chose to try to force upgrades for senior most qualified FOs like the regionals have done for years, or even junior FOs who just reached 1000 hrs with the company.