Yes, as long as you retain the copyright of the code you wrote (i.e. didn't explicitly assign it to someone else, or write it as a work for hire) then you can relicense it under whatever you wish at any time. You could make it available under GPL, CDDL, MIT, and the Beerware license all at once.
This stuff is relatively simple at its core. Copyright says, "You may not redistribute this work without permission from the copyright holder." An open source license says, "I grant you permission to redistribute this work, as long as you follow these conditions." Relicensing is just a matter of making another statement like that with different conditions.
If you die tomorrow, then the copyright transfers to your heirs, who could then do all the stuff you used to be able to do.
If there's no clear owner, then life becomes interesting. If there once was a clear owner who released the stuff with a license, that license is still valid, but relicensing isn't possible unless someone can demonstrate that they're the owner. If there was never a clear owner then you can't really use the stuff, although if you're brave you could proceed under the theory that if nobody claims ownership there is nobody to sue you.