RJ11 extension cable in place - conver to RJ45 (100Mbps)?

Hi guys, there is a place where they already have a few extension telephone cables laid out - the ones from DSE / The Warehouse etc. Is it possible to convert them to RJ45 and use them for computing?

They then can be plugged into the router and the other end plugged into laptops.

100Mbps is fine.

Saves the work of buying new cable and laying them out again. Very old house nearly 100yrs.

Thanks.