You shouldn't have to unstitch the webbing to get the new buckle over it. I think too that you can get just the plastic part and it just slips over the metal part and then you thread the webbing through it.
Probably fits tight but there should be no reason to have to take out the stitching.
Of course I have been wrong before...

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique