It's 5.5e. It's still 5e, as much as 5.0 is/was 5e. Just like how 3.5e was as much 3e as 3.0 was. It's a bigger change than, say, Essentials, which actually was 100% just an alternate slate of classes (with specific rules for how that new slate could be blended with the old slate). It's inarguably smaller than past full-edition changes, with the one arguable exception of 1e->2e. Certainly, it's a smaller gap than 2e->3e, 3e->4e, or 4e->5e.
Various core mechanics have changed. A number of things that used to be class features have become spells. A number of spells have been reworked. Advantage, which is a pretty deep core mechanic, has been expanded. Etc. It's not absolutely perfectly the same game. But it's also not so radically different that it isn't mutually intelligible, outside the specific bits that require conversion.
It's 5.5e.