PARF is expected to be 'lost' the moment you decided to continue driving, regardless if you going to change new, second hand or COE car. (keep buying COE car is different, another conversation). The only time you truly get back the rebate is when you stop driving 4ever.
The last 2 yrs of PARF rebate delta is actually small (5%), don't bother compare.
If you want to compare COE. then is current COE balance vs the 'renewed' COE yearly cost (linear depreciation for COE). Then again, no one knows what the price will be next yr. Maybe it's negligible (per year).
Sometimes the big bulk of the cost comes from the "implicit" body value. e.g. COE car price consists of 'body value'(A+B) + COE(C), where body value is implicitly correlated from market forces (A) (2nd hand dealer mark up) and PARF rebate (B). It's 'implied' for the simple reason no dealer will sell COE cars at COE+small value and suffer a loss from the PARF rebate when they take over, a better car with higher PARF rebate will always command higher delta else the second hand dealers have no incentives.
To illustrate: if you compare Jap cars vs our BMs. Maybe Jap COE priced at 60k, BMW at 75k, CAT B.
The constituents of the jap car likely to be 5k Body, 14k implicit PARF rebate, 41k COE.
BMW likely to be 12k body, implicit 22k PARF rebate, 41k COE.
with this, it's easier for you to understand that if you change BMW to COE Jap, you save alot because your implicit 22k gets carried forward to cover more than the Jap's. Problem is the implicit for jap cars for the next 10yrs might depreciate faster.
If you change BMW to COE BMW from someone else. you have to pay more (12k). But if it's yours in the first place, instead of buying another... it's technically your own money saved.
That said, personally i don't think all BMW models are suitable for renewal... some models are just better/lousier than other, do take that into consideration.