Your SSN does not expire; to my knowledge the only time you get to change your SSN is if you're in a Witness Protection Program. I know someone whose identity was stolen and she was not able to get her SSN changed. (That may have changed since though; this was 4-5 years ago.)
With credit cards it's tricky. If you open a CC before you leave, you can keep it, but it will be tricky when it comes to renewing the card. Usually there is a policy in place that only allows them to send a new CC to your home (existing) address. This is to prevent fraud; from their perspective it makes sense.
You can continue have a bank account, but again, if you have a debit card attached it can be difficult to get replacement cards in the absence of a US address.
There are some mail-forwarding services that allow you to have a US address that is not a PO box, but AFAIK sending/receiving cards overseas can be difficult. One of my colleagues was visiting from Europe and his company-issued Visa card expired during his visit. They had a really hard time sending him a replacement card. The US requires that the card be deactivated while en route and the company needed to provide a letter from the issuing bank certifying this. I don't know if the rule applies both ways, but this kind of rule would prevent a mail-forwarding company from forwarding you a credit/debit card.
The credit history does not disappear easily. If you have on-going activity (a credit card, a loan that you're paying off or some other debt), this will keep your credit score active.
It's hard to get a credit card in the US without some sort of income statement, and generally they want US tax returns, W2 forms, stuff like that. I don't know if it's possible to get a CC in the US based on income earned abroad, but if it is I can't imagine it would be easy.