Type conversions

Since StringReplace works on lists, I would use:

List @@ StringExpression @@ StringReplace[
    {"abc","def","2","ghi","7"},
    n:NumberString :> ToExpression[n]
] //InputForm

{"abcdef", 2, "ghi", 7}

Here is a benchmark of the existing answers.

make = RandomChoice[{"abc", "def", "2", "ghi", "7"}, #] &;

bob[lis_] := (lis /. n_?(NumericQ[ToExpression[#]] &) :> ToExpression[n]) //. {s___,
     str1_String, str2_String, f___} :> {s, str1 <> str2, f}

halirutan[lis_] :=
 Module[{isNumber},
  isNumber[s_String] := StringMatchQ[s, NumberString];
  If[isNumber[#], ToExpression[#], #] & /@ StringJoin @@@ SplitBy[lis, isNumber]
  ]

hubble[list_] :=
 Module[{res = StringJoin /@ Split[list, (LetterQ[#1] && LetterQ[#2]) &]},
  Table[Which[DigitQ[res[[i]]], ToExpression[res[[i]]], True, res[[i]]], {i, 1, 
    Length[res]}]
  ]

carl[lis_] := 
 List @@ StringExpression @@ StringReplace[lis, n : NumberString :> ToExpression[n]]

mrwiz2[lis_] := 
 List @@ StringReplace[
   StringRiffle[lis, "!"], {"!" -> "", d : DigitCharacter .. :> FromDigits[d]}]

Needs["GeneralUtilities`"]
BenchmarkPlot[{bob, halirutan, hubble, carl, mrwiz2}, make, 5]

If a string is a number can be tested by

isNumber[s_String] := StringMatchQ[s, NumberString]

and using a custom version of join that takes care if it is a list of numbers

join[s : {_?isNumber, ___}] := Sequence @@ ToExpression[s];
join[s_] := StringJoin[s];

you can apply SplitBy to collect all non-numbers in your list and convert the rest to integers:

join /@ SplitBy[{"abc", "def", "2", "ghi", "7", "8", "jkl"}, isNumber]
(* {"abcdef", 2, "ghi", 7, 8, "jkl"} *)