What change to a efi motor do you need to also re flash the ecm
LIKE EXHAUST ,,DISTRIBUTOR,THROTTLE BODY

Not sure exactly what you are asking here

Also we'll need to know your exact engine model, year, serial number if possible and ECM type?

The more info you could provide us, the better we might be able to help you.

If your asking at what point do you need to re-flash the ecm during modifying the engine then the answer is "it depends"...I know that's not what you want to hear but its not cut and dry with EFI/computers when they monitor sensors on the engine. There are small things you can "get away with" without flashing the ecm...but for the most part any change needs to be addressed by flashing the ecm to get the HP out of the change you are looking for.
Small changes like a cam that's "slightly" bigger then stock you may be able to pass off and not flash the ECM...you could probably change heads and the engine wont know the difference....and maybe even slightly larger injectors. You wont get the full HP its capable of but it will work....but start adding exhaust and it will mess with things...add throttle body then absolutely....guess it all boils down to how "FAT" the engine was tuned from the factory and how much of that extra you can use up before it starts going too lean.

****full disclosure***** i by no means am an EFI expert...but ive been playing with EFI live lately on my own vehicles and have a pretty good understanding of the grand scheme of things

I get what you are saying , so before I make a change check with someone if a re flash is needed,,,I change me exhaust from stock to the mercury like gils an saw no change at all in performance,,the motor is a 2000 efi mag,,I'm just doing the small changes not looking for anything big,but when I do make one ,I want to do it right or just leave it alone

Im no expert also on the marine side of things but engine performance fundamentals are the same air, fuel and timing makes power. Making small changes doesn't net much performance because hp gain is small so really not noticeable. If doing stuff yourself making small invest in nice wideband system to make sure your changes doesn't lean out motor. Also when doing ecm flash do as much changes you can b4 sending ecm to flash this way you dont be sending ecm back everytime you change something that requires ecm flash

That's why many have turned to MEFIBurn or different programmable computers all together...and obviously learning how to use them.

A wideband is great for A/F ratios, but not too effective for timing changes that your computer may be making that you won't be aware of.

Any EFI engine builder can advise you of this. Mark Boos at Precision Marine is great with ecm programing and tunned my previous 500 efi's for the new owner. It cleaned up the transom soot and picked up another 100 RPM.

Do you have a 454 or 502 EFI Mag engine? Did the exhaust you added have O2 bungs welded in? You can't really make accurate tuning decisions without reading the air fuel mixture in the exhaust. Where are you located?