Hello Thunkers !
I just want a closing to this topic, so here i am, telling you how i solve my issue, after a hard-time workaround
I had an application, lets say appA. I was not very happy with it, considering that it was my first application, so I used thunkable resources in a not exactly optimized way. So I decided to rebuild it from scratch.
My first mistake was that I did not start from a copy of the application (lets call it appB), believing that the key import from appA to appB in the end is enough to upload appB on GoogleStore as an update of appA
Well, it wasnt, and GoogleStore rejected my bundle, saying that appB has not the same fingerprint.
So, back to work.
I made a copy of appA (lets call it appC). Deleted all from it, let only a blank screen there. (tables, datasources, images included). Exported every sceen from appB, saved in MyScreens. Then imported in appC screen by screen from MyScreens those screens (the thing that helps alot is that saving a screen in MyScreens saves blocks from that screen too).
So now i have appC with all the screen imported. Unfortunatly this is the point when real work begins, because every imported screens created copies of assets, tables and so on, so you need to start cleanning and repairs errors, linking again assets to screens and so on.
My second mistake was to delete screens from MyScreens tab (i just want to keep my environment clean .Didnt noticed at that time, but that deleted pictures from appA aswell (dunno why, but that happened). So when i needed to recover assets as pictures to upload in appC (thank you @muneer and @domhnallohanlon for advices), i couldnt, cause appA didnt had any images to export )
So back to get images from primary sources, editing again… but in the end, after imported original key from appA to appC, i could upload appC as update of appA on Store, so this is the only thing that matters
Many thanks to @muneer and @domhnallohanlon for support.