Dont know whether I need to do anything but maybe someone could advise me.
I had an old laney 4x10 bass cab. the 60w hh bass drivers were a bit tired and so a picked up 4 celestion bl10-100's really cheap.
As the vent is just a single slot running across the front do I need to open it up for the more powerful drivers?
As ever it is not as easy as that. Reflex cabs are tuned and just swapping speakers without retuning the cabs is a bit like changing strings without adjusting the pegs.

Changing the slot changes the tuning not the amount of air pumped out.

If you tell me the internal dimensions of the cab I could work out the size of the slot for you to tune to the celestions. Alternatively if you simply block the slot (it needs to be airtight) you will probably get an ok sound.
The cab has internal dimensions of 22" x 22" x 11".
The speakers are 4 x celestion BL10-100