Just on top what druuna said:
If you want to make permanent changes to the file, you can either:
a) use the -i flag (the existing content will be overwritten):
Code:
sed -i '1,/^Initialization completed/c Initialization completed' infile
or
Code:
sed -i.bk '1,/^Initialization completed/c Initialization completed' infile
In this case a backup file (infile.bk) will be created with the original content.
b) redirect the output to a new file
Code:
sed '1,/^Initialization completed/c Initialization completed' infile > newfile