First, it'll be legally challenged by the mayor/city council.....however, the text of the original understanding says that the federal government has ultimate control (funding) and that they are allowing local management.
So I would go and suggest three things very likely to unfold quickly: (1) everything east of the Anacostia River (roughly one-quarter of DC) reverting to Maryland, (2) everything west of Rock Creek and Potomic Parkway (the far western one-third of the whole district, to include Georgetown)....also goes back to Maryland, (3) Trump likes to renames things....so I think the District of Columbia would be renamed 'New Columbia'.
For what remains? Trump might offer up a Constitutional Amendment....saying that one electoral vote will be added for the region (minus the farwest, and the Anacostia region).
As for daily management? I think he will agree to a commissioner with a two-year term, with a 2-star general to head police and fire protection (under the Corps of Engineers).
Locals shocked? Oh yeah.....this will turn into a massive legal fight.