I'll break it down for you
Any time at all, as long as you are willing to trade in and financially a different car works for you, then you can do that at literarily any point during your agreement (excess millage etc may apply)
You have a 4 year agreement, say you are 19 months in (random example) you go into the dealership and they have the car you like on offer/great deal. They calculate how much your car is worth and how much your settlement figure is. You are then in a position of negative equity (owing more on finance than the car is worth) or positive equity (being able to carry over the remainder as a small deposit).
You then work the figure on a new car and order. Collect, hand old one back. Agreement is paid off and a new one is started, simple.
My agreement ends on the 30th june 2021. If (big if) i was to keep the car for the full term, I would start making plan for a replacement (if ordering new) around 3/4 months beforehand. This gives you enough time to pick a spec, settle on a deal and more importantly, get the car built and delivered. My original thinking was as follows:
Car delivered on 30 June 2017 (6 months into overall production)
LCI predicted 42 months later (based on normal BMW timelines and F10 in particular pre LCI being produced between Jan 2010 and July 2013)
Order replacement LCI around month 34 of my PCP to hand car back around month 36 coinciding with LCI and end of warranty
I now would personally not keep one a day out of warranty and think I will be changing cars much earlier than month 36 in any event.
What you are talking about here is voluntary termination. This can be triggered once 50% of the entire loan is pad back (you must remember that the bubble will remain in that figure. I.e if your purchase price was £42k the bubble (guaranteed future value/residual) will be around 40% of that at £16.8k. To achieve 50% repayment, you have to pay off at least £21k i.e half of full amount.
You have to remember that any positive equity will be gone on a "VT". On the flip side, legally, if you have gone over your allowed millage you can hand the car back and not suffer any penalties (except for wear and tear) under law as long as you don't sign that you accept any excess charges - which the finance company will insist on. If you are not taking the piss by handing back a car after 3 years with 60k miles when you were only supposed to do 8k a year, you'll likely get away with it.
Setting a high guaranteed future value in future deals means it will take longer to reach the VT 50% point as repayments are smaller on a monthly basis. Exercising right to VT should not mean that BMW finance for example will refuse to give you another car in the future.
If you want to decrease costs, now will probably be a bad time, 5 series are at an all time high of 4.9% APR. Having said that, if you got a small discount on your current car and the deposit was not to high, it may be worth having a browse through CarWow etc. Getting a good discount on a new car even with slightly higher APR may be able to decrease your monthly payments especially if you are under your mileage allowance and leading up to March 2019 as dealers will be doing cracking deals to register as many cars as they can on 68 plates and getting rid of new registered stock. Haggling on the APR is also possible with BMW finance.
Edit: Alternatively, the current 3 series is on cracking deals at the moment with 0% APR across the board. I know one person who got almost 40% of a 320d Shadow Edition with Pro Media, front and rear PDC and reversing camera (was a car ready in stock) and another who got almost 30% off on a very wel spec'd 340i on a short 24 month contract