About to RMA a failing hard drive that is now working

WD 750GB 2.5" Scorpio Black, coming up with bad sectors and blue screens. WD Data Lifeguard stopped mid way through Extended Test saying too many bad sectors. Started RMA process with WD and gave credit card details so that they could send me a replacement drive ahead of me sending the old one back. If I fail to send the old one back within 30 days, I get charged. New drive arrived and installed.

I then proceeded to wipe the old drive prior to sending it back; I used a 3 stage process. The old drive now passes the Data Lifeguard Extended Test, although the fast number counting during the 3 hour test would occassional stop for a second or two and then continue.

What do do? Is the old drive now OK? or do I send it back under the RMA anyway?

I could just keep the old drive and be charged, and thus have a spare drive.

Another twist: The old drive warranty was to 2017. The new drive warranty is only to 2013.

If I proceed with this RMA, I will finish this process with a drive that has very little warranty left, which is very annoying.

What do I do?

i would complete the RMA, and see whats needs to be done to  put the warranty of the old drive into the new one