For loop in bash to replace string in text file

I need to generate new files by modifying a string in an existing file.

I have a text file called newheader.txt, it looks like this:

@RG     ID:$FILENAME     SM:$FILENAME

I also have a text file called filenames.txt that looks like this:

ZF-150-81_S49
ZF-150-82_S55
ZF-150-83_S59

I would like to have a for loop to go through filenames.txt one line at a time and replace the $FILENAME string in newheader.txt with the line in filenames.txt and make each of these into a new .txt file with that line in the title.

This would create three output files. For example the first output file would be called ZF-150-81_S49.header.txt and inside it should look like this:

@RG     ID:ZF-150-81_S49     SM:ZF-150-81_S49

In other words something like:

for $FILENAME in filename.txt; do replace $FILENAME in newheader.txt > $FILENAME.header.txt ; done

I have done bash for loops but not where it requires replacing a string in a text file. Happy for other solutions such as perl if this makes more sense. Thanks for your assistance.