I guess I would throw it back to the slashdot team and ask, what is it you hope to achieve with rebranding? Why a new logo? Is it merely to commemorate the history of slashdot? If so, I'd say you've already done that with the month-long logo fun.
Rebranding is done for a number of reasons. Historical milestone isn't typically one of them. It's sometimes used as an excuse for "We haven't grown our user base for a while and need something fresh to entice more visitors", but the event alone isn't usually enough. This is generally true because if you are happy with your growth and retention, don't mess with it.
If you aren't happy with growth or retention, that could be due to any number of factors and a new logo isn't likely to fix it. Knowing who the target demographic is would be the key to rebranding. But I advise extraordinary caution. Slashdot is old-guard when it comes to the internet, an institution really. An unchanging logo conveys stability and reliability. It also helps smooth transitions to other changes you may make in the future with respect to appearance, functionality, or content.