They can't hold on forever in 1914....so they will have to change that. They did for 1874, the supposed invisible presence of Christ, it was changed in the mid 30's, so it is about time for a new light, don't you think?
I don't think they'll ever change 1914, maybe give it a new meaning. They may revert to the old interpretation of a separate beginning of "Last Days" and beginning of parousia, like what Russel believed, leave the Last Days on 1914 and count the generation from the parousia, the beginning of which can be put to an unsignificant event as a local convention somewhere (remember, major prophecies in Revelations are said to be fulfilled by underwhelming accurances).
So no, 1914 will be staying with us in one form or another. It's the only 'prophecy' they sort-of got it right - by this I mean they got the year right, but they totally missed on what will happen on that year - it's still better than the rest of their attempts to prophesize.