Motortrend has a sort of article highlighting some behind the scenes upgrades of the M3H over the old M3. One can extrapolate to some degree of the updates applying to the MY Juniper. I think the most impressive is the suspension bits. It soaks up undulations and road cracks very well without compromising sporty driving. All around acoustic glass. Overall fit and finish improvements.
Oh and adaptive headlights, BLIS, sexy one-piece C taillights, camera on-demand, stupid wipers, Tesla wants to kill you stalkless, angry bird vs Kermit front fascia, awesome perforated and ventilated vs pudgy couch front seats, French stitching vs. single cheapo stitching + other stuff.
Munro Live has a video on the behind the scenes upgrades.
After driving the M3H, I find the OG M3 quite dated. It might not look it now, but I feel the current MY will accelerate in age once Juniper shows up. Only you can determine this would but your or not.