Find each instance and replace with unique value in string

Question

I have some text in a string as below:

{121:SOMETHING1}}{4:
.
.
.
{121:SOMETHING2}}{4:
.
.
.
{121:SOMETHING3}}{4:

I want to sequentially find and replace the value between 121: and the first }. I can successfully do this using the following code in C#

       var rx = @"121:(?<value>[\s\S]+?)}";           
       string temp = tag121;

       string stringToChange = //as above
       for (var m = Regex.Match(stringToChange , rx); m.Success; m = m.NextMatch())
       {
           temp = generateUniqueValue()

           stringToChange= Regex.Replace(stringToChange, m.Value, temp);

        }

However, if instead of having different values between 121: and } I have exactly the same value for all the tags e.g instead of SOMETHING1, SOMETHING 2 etc, I just have SOMETHING for all the lines, then this code does not work. It ends up setting just one value for all the lines instead of unique values for each.