As soon as you change the retention level on any image it gets applied to all of the files, which are part of this image irrespective of few files or millions of files.
What you have intially asked, how to change the retention on the backup which is in progress and the answer is --- You can not change retention on the backup currently in progress. You can change the retention only after it completes successfully or partially successful.
Your backup is running for 2 weeks, which means if you need to recover the whole backup set it will take longer than 2 weeks and the question comes can you afford a restore to run for longer than 2 weeks.
You should revisit the approach you have taken to backup this data, if flashbackup is not the solution then you may want to split this backup in multiple pieces (may be at directory level) or make the client as SAN media server or make it a SAN client.