test1,test2,test3
test4,test5
test6,test7,test8,test9,test10
test11,test12,test13,test14

test1,test2;test3
test4,test5
test6,test7;test8;test9;test10
test11,test12;test13;test14

$ sed 's/,/n/g; s/n/,/; s/n/;/g' input
test1,test2;test3
test4,test5
test6,test7;test8;test9;test10
test11,test12;test13;test14

$ sed -e 's/,/;/g' -e 's/;/,/1' infile
test1,test2;test3
test4,test5
test6,test7;test8;test9;test10
test11,test12;test13;test14

sed 's/,/;/2g' infile

awk '{gsub(",", ";"); sub(";", ","); print}' file.txt

% cat file.txt
test1,test2,test3
test4,test5
test6,test7,test8,test9,test10
test11,test12,test13,test14

% awk '{gsub(",", ";"); sub(";", ","); print}' file.txt
test1,test2;test3
test4,test5
test6,test7;test8;test9;test10
test11,test12;test13;test14

txr -e  '(each ((line (get-lines)))
           (set [line (rest (where (op eql #,) line))]
                (repeat ";"))
           (put-line line))' < input

$ txr
This is the TXR Lisp interactive listener of TXR 148.
Use the :quit command or type Ctrl-D on empty line to exit.

1> (take 3 (repeat ";"))
(#; #; #;)

2> (let ((a "a,b,c"))
     a)
"a,b,c"

3> (let ((a "a,b,c"))
     [a '(1 3)])
",,"

4> (let ((a "a,b,c"))
     (where (op eql #,) a))
(1 3)

5> (let ((a "a,b,c"))
     (set [a '(1 3)] (repeat ";"))
     a)
"a;b;c"

6> (let ((a "a,b,c"))
     (set [a (where (op eql #,) a)] (repeat ";"))
     a)
"a;b;c"

7> (let ((a "a,b,c"))
     (set [a (rest (where (op eql #,) a))] (repeat ";"))
     a)
"a,b;c"

8> (let ((a ""))
     (set [a (rest (where (op eql #,) a))] (repeat ";"))
     a)
** replace-str: "" of type lit is not a modifiable string
** during evaluation of form (sys:dwim-set a #:g0145 #:g0144)
** ... an expansion at expr-8:2 of (sys:dwim-set (#:g0146) #:g0145
                                                 #:g0144)
** which is located at expr-8:2

9> (let ((a (copy "")))
     (set [a (rest (where (op eql #,) a))] (repeat ";"))
     a)
""

10> (let ((a "a"))
     (set [a (rest (where (op eql #,) a))] (repeat ";"))
     a)
"a"

11> (let ((a "a,b"))
     (set [a (rest (where (op eql #,) a))] (repeat ";"))
     a)
"a,b"