(1) Retroactive use won't be allowed; (2) you probably won't get an upgrade on a SIN segment; (3) don't listen to anyone who tells you to look for cash; (4) Don't expect anyone to take an expired cert.

But, I think if you explain that you saved them for a DEL trip AND could have used them on another trip but spent 50K mi instead (but would now like the experience of trying UA to another destination), you may get them extended. Good luck.