Yes, older coolant will be left in the engine block if all you do is drain the coolant via the petcock. Unless you’re in a “stranded on the side of the road” situation, there’s virtually no upside to mixing coolant brands/types....Also coolant change: Going to do drain and refill (no flush), will that leave some old OEM coolant in the system, because I am going to use dexcool AF888, 2 diff coolant will get mixed in is that not ok or OEM coolant only got very little left and no need to worry.
If I were only doing a drain & fill, I’d refill with only the exact type of coolant I drained.
However, if you’re going to do a full on flush using distilled water then it’s OK to change brands.
Be sure you use the correct OAT/HOAT type that’s appropriate for your vehicle. Also don’t forget to refill with a 50/50 mixture rather than undiluted coolant.
This is a good coolant resource chart for Ford vehicles.