Escape metacharacters
You probably forgot to quote metacharacters. For example
xxx\,xxx not xxx,xxx
xxx\\xxx not xxx\xxx
Example 1
In the sed expressions I have replaced \ with \\ and , with \, to indicate that these are to be interpreted as a literal character and not as a metacharacter with special meanings.
$ echo 'foo AEP/e2cy//rSq,W8VJu\TMb1vGeeyxcr bar' \
> | sed 's,AEP/e2cy//rSq\,W8VJu\\TMb1vGeeyxcr,/uK2S9v6A\\NnW\,PbRB+WUwn/Uu83\,Gym,'
foo /uK2S9v6A\NnW,PbRB+WUwn/Uu83,Gym bar
You cannot use an unescaped comma in a search string or replacement string if you are using commas to delimit them. But you can use a different delimiter to make life easier
$ echo a b,c d | sed 's!b,c!xxx!'
a xxx d
Example 2
I don't see why you had a problem with this.
$ echo foo 8000::/1 bar
foo 8000::/1 bar
$ echo foo 8000::/1 bar | sed 's,8000::/1,128.0.0.0/1,'
foo 128.0.0.0/1 bar
A well written question should include real examples of actual commands, their output and actual error messages. Use cut & paste.