Hi there! I bought a 2018 450i in March. I really love the machine so far, but yes, the shifter can be difficult to move at times. I find this really only is prevalent when you are on an incline. It seems that when the machine is in gear, and gravity is putting pressure on the drive shaft, the transmission doesn't want to shift. However, if you rock the machine back and forth a little bit it will usually shift just fine. I literally just wiggle back and forth in my seat to do this. If you are in a tight spot, use the throttle in whatever gear it is in to accomplish the same. For example, I had it on my trailer the other day and it didn't want to come out of reverse into high. So, what I did was give it a little throttle to move the wheels ever so slightly against the back of my trailer, put my other foot on the brake to hold it there, release the throttle and then it shifted just find. Again, anything you can do to just take the pressure off the drive shaft will allow the machine to shift.
Another helpful tip: when you park on an incline, just leave the machine in gear and engage the manual parking brake. Many users say the ONLY time they ever put the machine in Park is when they trailer the vehicle. I personally also use park when I park the machine in my garage (on a level surface). But when I am out riding, I never put it in park. I just use the manual brake.
One thing other have mentioned is they find their machines shift more easily with synthetic oil. I just did my first oil change yesterday and put full synthetic in. So, I'll update this again if I notice any improvement in ease of shifting.
Good luck with the new machine, and let us know how you're liking it. I've got 112 miles on mine now and been having a lot of fun. I also came up with a couple little tricks to quiet the back end that I'll share in another post. I don't know if you've noticed it yet, but that rear tailgate storage compartment rattles like a son of a gun, as do the little cable connectors that hold the tailgate cables. Both are easily remedied.