For the record changing subclasses as a result of roleplaying and consequences is huge and minor at the same time.They could in 4e Essentials - there were intra-class multiclassing feats that let you gain subclass features of another subclass of your class. So Protectors (summoner Druids) could take the feat for the Sentinel's animal companion or the 2008 Druid's Wild Shape, and vice versa versa.
I'd really like to see something like this come back.
That said, there ARE 5e suggestions for changing sub-classes wholecloth - it's in the 2014 DMG, and built into the example of a Paladin Oathbreaker subclass. You break your oath, and instead of losing all your abilities until you repent, your subclass is replaced with the Oathbreaker one.
And on thinking about it that's one thing that could be very interesting - multiclassing through subclasses. Each class offers one "multiclass version" that you take instead of an own-class subclass.