Changing tempo project, without affecting the MIDI items tempo

Is there any way I can do that?

Yes, with the item(s) in question selected hit CTRL + F2 or find "Source properties..." in the item context menu. There you tick the box labeled "Ignore project tempo information, use:" and insert the desired tempo and time signature in the boxes below.

Alternatively if you want the items to keep their positions in time 

Simply turn the regarding MIDI-tracks timebase to "time" before changing the tempo (right click on track). In case you want all tracks to remain in the same tempo change the project timebase to "time" and leave the tracks timebase at their default.

thank you! the "Ignore tempo project information" works perfectly...on the other hand, changing the timebase to "time" doesn't seem to work, because the midi items keep changing time when I change the project time

Yep. That's because MIDI files are beat based per definition. The timebase settings will only affect how the start times of the individual items behave with tempo changes. They won't free up the content from project tempo information.