Programming :: Replace A Value In Order?

I want to change the parameter for ThreadLimit for one file as shown below using sed. I want to ignore the commented line. But the command I am using also executing with commented line.

# ThreadLimit: maximum setting of ThreadsPerChild
ThreadLimit 25

I want the desired output as

# ThreadLimit: maximum setting of ThreadsPerChild
#ThreadLimit 25
ThreadLimit 50

For this I am using command.

sed '/ThreadLimit/{;h;s/^/#/p;x;s/ .*/ 50/;}' test.txt.1 > test.txt