Can i change my seat ?

Hi I am booked on a flight to Barbados in August: the BGI to LGW seats are great upstairs, but the LGW to BGI seats are not so good.81 D E F G or are they good seats!!!
As certain seats are released on the day:
Can I check in online and change my assigned seats even though mine have been assigned?
Shall I do twilight check in and change my seats at the airport if possible?
Or shall I just check in on the day early at the airport and change my seats?
In a roundabout way I suppose I am asking what the best way to change my assigned seats
Of course the other hope is as the VS 29 seems to be operated a lot by LHR aircraft in which case less PE and more [:?]
As certain seats are released on the day:
Can I check in online and change my assigned seats even though mine have been assigned?
Shall I do twilight check in and change my seats at the airport if possible?
Or shall I just check in on the day early at the airport and change my seats?
In a roundabout way I suppose I am asking what the best way to change my assigned seats
Of course the other hope is as the VS 29 seems to be operated a lot by LHR aircraft in which case less PE and more [:?]