Hi,
I will start with a disclaimer 😃 : uSync.Migrations is very much not a point and click solution for migrations (unless you're lucky) and depending on local setup etc, there will prob be issues things might not work. the whole project is open to additions to fix any issues etc. We don't offer "uSync" style support for migrations because of the complexities of upgrades etc. and because its a side hack project really.
but..
I would say probably the best way to go from grids in v7 -> block grids in v13 is to do it in a few steps checking and re-exporting as you go.
1. as above upload the v7 do a migration of everything into the site. and check that looks right.
2. in the normal uSync dashboard run a full clean export of the now converted site. (this will get you a clean uSync/v9 folder to do the conversion on.
3. Create a new conversion project in uSync.Migrations - pick the convert plan, run and fingers crossed.
if things don't work its probably worth digging into the 'migrate' folders in uSync and looking to see if you can see any block grid stuff in the content.
also - because its often super slow, i rename the uSync/v9/media folder to something else first time around, if you have lots of media you don't want to be converting it every time.
hope that helps. the repo issues
https://github.com/Jumoo/uSyncMigrations/issues might have some other pointers issues people have had. and the code is there too. if you want to dive deeper.