I tried to change the config file to set old UI as default but not working

Hello.

I tried to change the config file to set old UI as default but not working. Or maybe I am doing it wrong.
What i did:

Added this to config file "traccar.xml"

entry key='web.old'>true</entry>

am I missing something?

Where do you get that parameter from?

I got it from https://www.traccar.org/configuration-file/

web.old config
Flag to enable or disable old Traccar web interface (true to enable, false to disable).

I must surely be using the wrong parameter / syntax.

That's a very very old parameter that shouldn't be there. That was for version 2.x. Removed it.

You actually just need to change the path:

<entry key='web.path'>./legacy</entry>

Thank you so much

Is there no way to change the interface without changing the configuration?
I would like to use modern most of the time and legacy a few times, but with 5.3 I am forced to use only legacy!

There's a way. You can set up a web server and host both apps at the same time.