I think Royal Enfield might have frinkled with the ECU between 2015 and 2019 to meet changing emission requirements.
As for the rest of the bike, Royal Enfield was always changing things to improve them.
Since 2009 they changed the wiring harness so it doesn't look like a rat's nest in the head light casquette, changed the fuses from the old glass tube to the modern push in type, added a second oil drain port under the crankshaft, changed the front fork design, changed the size of the front rim, changed the direction the crankshaft bolt that holds the primary drive sprocket on so it wouldn't unscrew itself, and a host of other improvements.
They never seem to reserve these changes for a "yearly change" for the new years models, but rather, do it whenever the mood suits them so some motorcycles made in a given year might have some of the changes but others that were made in that same year don't.