Roseland
The answer lies in the en-route contingency fuel (typically 3% of planned trip fuel) which has to be loaded as reserve together with fuel for holding at destination and diversion to alternate.
A first flight plan is filed with e.g. Bordeaux as "destination" and anywhere suitable as alternate. Could be Toulouse, could be CDG.
When getting close to BOD, if the 3% contingency has been consumed you land at BOD.
If, as is usual, most of the 3% has not been used, you file a second flight plan from this redispatch point to CDG.
Fuel for holding at CDG and diversion to ORY is still required but contingency fuel is now only 3% of the planned burnoff from reclearance point to CDG (very little). Very very common and perfectly legal.