One disk on my Synology 2413+ started with bad sector counts some time ago, and since 3 days it gives me I/O errors.

I get an health report every first day of the month, and since the 1.08.2014, disk 1 has 1 bad sector count.

Disk Reconnection Count 0
Bad Sector Count 1
Disk Re-identification Count 0
I thought that it might be not so bad, and it also stayed like this for a while. But in the health report of January this year, the bad sector count had increased to 50. One month later it was at 83, where it stalled until this day.

Unfortunately since last Saturday I also get I/O errors from that disk, and that once a day:
Dear user,

The hard disk 1 on Synology2413 had an I/O error, but it is working properly now after several retries. It might have been caused by bad sectors. If this error occurs again, please back up your data and run the S.M.A.R.T. test on your hard drive to examine the hard drive status.

Synology DiskStation
S.M.A.R.T. test is normal and doesn't show anything unusual, status is green.

As the bad sectors get locked, and at the moment the count is not increasing, should I be worried? Do you think I should change the disk anyway?

You know that I'm running the NAS as one single volume in Raid 6 mode. Could I also use f.ex. a WD Red disk as replacement, as I'm using 12 x 4TB Hitachi Deskstar™ 5K4000 (HDS5C4040ALE630), which are not available anymore, does that matter?

Here's an excerpt of my log file: