This can be done with this, 

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

If you have GNU `sed`, you can also use,

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