First isolate lighting circuit for upstairs lighting then secondly disconnect wires from inside pullcord. If pullcord has three wires and earth then this is two way switching. You will need to take switch off wall and disconnect the same three wires out of the switch and tape up the ends at the switch and at the pullcord the connect the two remaining wires in the switch at terminals com and l2 for one way switching
Hi presume it's on a 2way circuit with sw. by door?
If so then [after isolating circuit] note which wire of the 3 connected to switch goes to term marked C or common. Remove sw. replace in loft with small 4 term junction box, putting common wire and either one of the other 2 (but not both) together in one term. put other wire in another term alone, and earth wire (if there is one) in another term alone.
If set up not as above reply with more info on wires connected at pull sw, (colours / terms).