Pretty sure they are going to move onto an new edition in a year.
I'm pretty sure they're going to spin it as 5.5e (just not by that
name, because calling it "5.5e" might upset people even if it's true.) My bet is "50th Anniversary Revision/Version" or the like, or as people used to call it before the "One D&D" playtest, "5.50e." They aren't going to change the fundamental math, but a lot of baseline stuff is going to be rewritten in ways that are not
quite compatible without significant oversight. You'll still be able to use the deprecated 5.1 SRD content, but certain aspects will be disfavored without alteration. E.g. the level at which subclasses come online may change for some classes (I expect them to try to normalize it to level 3), spell lists are being
heavily rewritten, several spells are likely to have their mechanics modified while still using the same name, "species" don't offer stat boosts but non-"species" character options do, etc. The conversion document won't be terribly long, but it will require
some conversion.
On the sliding scale of edition changes, it will be bigger than the
not actually a change at all 4e Essentials (which was literally just "here are more options, they don't replace or overwrite anything, they're just other options"), comparable to the 1e->2e or 3e->3.5e shift, and smaller than the comparisons most people make between 3.5e and 5e (that is, "different games that are really similar, enough that you'll get tripped up on the places where they differ" or the like.)
The actual dynamics of play and building-block mechanics will change very little, but their structure and interrelation will change meaningfully, and individual bits and bobs may change significantly.