Separate string at upper case letters using StringReplace

Simply use the pattern Except[WordBoundary, t_?UpperCaseQ] as the pattern to be replaced:

sR = StringReplace[Except[WordBoundary, t_?UpperCaseQ] :> " " <> t];

sR["AbcDefGhi"]

"Abc Def Ghi"

sR["DistrictOfColumbia"]

"District Of Columbia"

sR["A B C"]
"A B C"

How about using StringSplit in combination with Riffle?

f[x_String] := StringJoin[Riffle[StringSplit[x, s_?UpperCaseQ :> s], {"", " "}]];

f["AbcDefGhi"]
f["DistrictOfColumbia"]

"Abc Def Ghi"

"District Of Columbia"

Unfortunately, it also does

f["A B C"]

"A B C"

So maybe this comes closer to your requirements:

g = StringReplace[
   RuleDelayed[
    Repeated[s : Except[WhitespaceCharacter]] ~~ (t__?UpperCaseQ),
    StringRiffle[Join[{s}, Characters[t]]]
    ]
   ];
g["AbcDefGhiAAA"]
g["DistrictOfColumbia"]

g["AbcDefGhi"]
g["DistrictOfColumbia"]
g["A B C"]

""Abc Def Ghi A A A""

"District Of Columbia"

"A B C"

Note that without s : Except[WhitespaceCharacter], the latter would have been "A B C".

What about?:

StringTrim @ StringReplace["AbcDefGhi", t_?UpperCaseQ :> " " <> t]