This has gone out over the announcement mailing list, and is also on their web site. But in case you aren’t checking those, seemed important to get people to see.
14 September 2015 — Human Genome Browser default changed to GRCh38/hg38
In conjunction with the release of the new 100-species Conservation track on the hg38/GRCh38 human assembly, we have now changed the default human browser on our website from hg19 to hg38. This should not affect your current browsing sessions; if you were last looking at the hg19 (or older) browser, the Genome Browser will continue to display that assembly for you when you start it up. There are circumstances, however, in which the selected assembly can switch to the newer version. For instance, the assembly will switch to hg38 if you reset your browser defaults. If you find yourself in a situation where some of your favorite browser tracks have “disappeared”, you may want to check that you’re viewing the right assembly.
We will continue our efforts to expand the annotation track set on the hg38 browser to include many of the tracks present on previous human assemblies. In cases where it makes sense, data may be simply “lifted” from hg19 using migration tools. In many instances, however, we must rely on our data providers to generate new versions of their data on the latest assembly. We will publish these data sets as they become available.
For a summary of the new features in the GRCh38 assembly, see the overview we published in March 2014.