How to upgrade project to different version

i created project using 4.17 and upgraded to 4.19.1 but now i keep m getting missing .dll error.
the project unable to compile dll
it doesnt work at all…
how to rescue my project without starting new one because this would be ■■■■■■■■ really
Im posting this to bug because it seems this error has been here for a while and no solution have been working
like this one

UE4 project code as any code naturally degrades due to changes in API that your code is using.

First of all in 4.17, open your unreal project in Visual Studio without opening editor (open sln file in you project directory) and rebuild your C++ code so it will be build from 0, look on compilation log (not error list, the Output tab) and look for any deprecation warnings. If you have any solve those deprecation warnings, those warnings usually have tips and hints what you need to do, you can always google to check how to fix perticilar issue, you might find update information explaining the change. and what you need to use now.

From my history of using UE4 some API changes happens suddenly without deprecation warning, in that case you will get standard compilation error and you need to investigate if there was change in APIs manually, if for example compiler saying that you use undeclered function or you have incorrect arguments in function, go look up API reference to see current version of function and correct your code according to that.

Once you done that your code is ready for update. Now best way to update you project is to do it version by version so first update it to 4.18 and repeat the process i mentioned, solve any deprication warnings. And you do that on each version, generally if you want to keep your project up to date you need to update it to newest UE4 version all the time, the longer you hesitate to do so the more update process became troublesome. The bigger jump in version the bigger potential of missing out deprecation warnings instructing you what to do and more things you need to figure yourself. Also recently there was changes in build scripts (*.cs) which is quite undocumented zone of UE4, so some changes really requires advance knowledge of C++ programming in UE4 to correcly solve the issue. If you have some C++ programmer then leave the code update job to him. If you got code from somewhere else you should contact creator of the code as it is there job to keep code up to date or else they dont do that for some reason.

I just anwsered other quastion which shows example of deprication warning, if that function i pasted would be used in C++ the warning would been displayed by compiler in logs: