Here is what I learned in my quest to deal with the 20 or so Metrocards I found around my house when I moved.
1. You can use up the small amount of a Metrocard on a bus and make up the balance with change. However, you won’t be able to use the card for a transfer if you do this.
2. If your Metrocard is not expired, you can bring it to any attended station booth and ask them to consolidate multiple cards or add the remaining balance to a different card.
3. You can choose to add money to the card at a Metrocard Vending Machine. However, if it is a month or less until expiration of the card, the machine will ask you if you want to transfer it to a new card.
4. If the Metrocard is expired, but expired less than two years ago, ask the station booth attendant for the form (with the accompanying addressed, stamped envelope) to send in with your expired Metrocard to get the balance from your old Metrocard sent to you on a new card, with a new expiration date. Then, you can consolidate, as explained in item 2. Warning: the MTA told me they were “three months behind” in processing these, when I spoke to them in March, 2012.
5. If your Metrocard’s expiration date was more than two years ago, forget it, you’re S.O.L., baby. Throw it away.
6. No, you can’t check your Metrocard balance online. Yes, you should be able to, given today’s technology, but you can’t. You can check it at a Metrocard reader in a station, though.
7. You can check your balance by phone IF you are enrolled in EasyPay. If you have read this far into this post, you are not enrolled in EasyPay. Forget it.
SYNTK 
If your Metrocard expired less than one year ago, you needn’t mail it in and wait the 3-4 months that it takes. A station booth clerk can transfer the money to another card, but only if it’s less than one year expired. If the card expired between one and two years ago, then yes, you do have to mail it in.
Good info! Thank you.
Thanks for the info will do that
Thanks, I have recently been hunting for facts about this subject matter for ages and yours is the best I’ve located so far.
I don’t understand why a metrocard needs to expire? This is so arcane.
Darren, agreed! There is so much about this system I just don’t understand.
my son is autistic and his card expired on september 2013, mine also expired on june and we never got a new one..I have made phone calls but i always get the machine to answer, never get to speak with a person.
Hi Edith. Thanks for reading. I have found it somewhat difficult to deal with the MTA on the phone, however, I have gone to their office at 3 Stone St. in lower Manhattan (here is a link to directions and info). It may take a while, but when you do actually talk to someone there, they are helpful. To me, they seem genuinely concerned about helping you. When the lines get a little long, I’ve seen a manager come out of the back and try to make sure people are in the line that could help them and/or have the information they need to get their problem solved. I don’t know if it’s doable for you to get to this office, but I have had luck there. I haven’t tried to do the things you are doing, so I cannot talk specifically about renewing your issued cards or replacing ones that never arrived. However, if you don’t have all the information you need the first time you go there, they will tell you what you will need. I can tell you, I am fans of the people who work in the MTA office at 3 Stone Street, because even if they can’t help you right away, they certainly try! Good luck!
Or just leave it on top of the “swipe here to check your balance” box for me to find it
http://whatyourdonotknowbecauseyouarenotme.blogspot.com/2009/06/on-metocards.html
Thanks for this! Should happen I moved and found four metrocards lurking in my possessions…