As others have already noted, there is no automatic tool that can do this for you, at least from the official developers of the platform.
The biggest changes were made to the form component, which has changed a lot since version 2.0. In general, I know that this is a difficult task, because you need to find and replace things, but in general it should be a matter of one day or half a day.
In addition, it is likely that you are not using all and all the features that have been changed since Symfony 2.0. This means that you will not need to change the code for each point of update notes.
In my case, I did for each bullet, first check if I used this function, and then went into search and replace mode.
It should go pretty fast. It's a long time, but you can do it :)
Good luck Matt
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