The Tristars are already on the US register. Clearly nobody is going to dismantle a VC-10 !
Tankers for the USN contract do not have to have an HDU - just at least 2 x underwing pods or 2 x HDUs for redundancy on a tasking.
Receiver clearances are in priority groups - with F/A-18 first - with bidders required to estimate times and costs to get up and running. There are specific payments available to get to IOC
I do see someone buying up the 4 x Luftwaffe A310 MRTT when they are up to speed with their new plot. They have already got the A400M MRTT doing the part of the role and delivery of the NATO A330 MRTT fleet is planned to be complete by end-2024.
https://www.nato.int/cps/en/natohq/news_177484.htm