Import gpx into Connect change distance and ascent

I add the two gpx: first is the good one, second is the one exported from connect.

ViaDellaLanaEDellaSeta.zip

Yes, Garmin Connect optimizes GPX files on import, and reduces the number of trackpoints. So for example your original GPX has almost 9682 trackpoints, while the imported course has only 2993 trackpoints. Many watches would not be able processing huge GPX files, and it would also impact their power consumption, hence Garmin optimizes the courses in this way. While it is usually perfectly OK for the navigation, there may be indeed deviations in the distance and elevation gain. Repeated small bumps may get ignored in this way, reducing so the total value. 

Thank for the answer! What I would expect is to see the real gpx on connect, with the real data, as on all the other apps and services. On Connect we are not in a watch but in their server, and even in the watch I would expect that instead of losing resolution it would optimize the trace by splitting it during import or without even me knowing that, or at least give me a warning when I'm importing it that the file is to big and he lose resolution.
If I didn't know the real data I could train myself for a wrong distance/altitude, not good...
Even during the navigation what data will I see with ClimbPro? Real one or the one he imported? If the second it would be a huge problem because I see fake data and I can run out of battery (mine) without knowing what is happening because I've done double the altitude...

If I split the gpx trace (to have the real data on it) Is there a way to change navigation trace inside the current activity without stopping it?
Thanks

Garmin optimizes the GPX files on import. Splitting them will probably not help much, but if you want to try it, you can indeed change navigation Courses during an activity.

But if it optimize every trace no matter how is big I can't have a good read with ClimbPro, I will always have fake data on it? It's unusable in the mountain