File conversion from MuseScore 2+ to MuseScore 3+

The layout algorithms have changed considerably to avoid the need for so much manual adjustments, but this does mean scores that relied heavily on manual adjustments to achieve a particular look will need to be re-adjusted. First thing I would do on import is Ctrl+A to select and Ctrl+R to reset everything (even if you choose the reset option on import, some things are not reset) then go from there.

In your case, the way you manually adjusted the slurs was especially prolematic. For future reference instead of dragging them below the staff, simply press "X" to flip them - both 2.3.2 and in 3.0. So, after the Ctrl+R, I would select that first slur and press "X" to put it below the staff. Also, the default settings for system text has changed (in large part to make it possible for them to also be flipped using "X", which was not possible in MuseScore 2), so I'd go in and update that. Click one of your system text elements and use the Inspector to set the offset to -2, the alignment to baseline, then hit the "Set as style" buttons next to those controls.

Score like this that relied a lot on manually positionally a bunch of text to get a certain look are the ones that will require the most work to update.