GetNthRecord is a function that can be used to access a field from another record in the same portal. I've used a simple calculation to move a record up one row in the portal:
Set Field [ YourPortal::SortField ;
Let ( [ R = Get ( RecordNumber ) ; // position of current portal row
R1 = If ( R > 2 ; GetNthRecord ( YourPortal::SortField ; R - 2 ) ;
R2 = If ( R > 1 ; GetNthRecord ( YourPortal::SortField ; R - 1 ) ] ;
(R1 + R2 ) / 2
) // Let
After re-ordering you can either use a looping script to renumber them to make the numbers pretty or you can use Go To Related Records to pull up a found set and renumber them with the Replace Field Contents script step.