Can I replace my range cord with one having larger conductors?
You can use a 50A rated cord for a 40A appliance with a 50A receptacle on a 40A circuit.  

Pay special attention to NEC 210.50(C) Appliance Receptacle Outlets. Appliance Receptacle Outlets...shall be installed within 1.8m (6 ft) of the intended location of the appliance. 

You will probably need to install a permanent breaker lock out device in your electrical panel to comply with NEC 422.30/422.33 that allows a receptacle accessible through the drawer to satisfy the required disconnecting means. 

Also check with the Installation Instructions of the range to see if it specifies receptacle location or cord length. Lab Listing (UL,CSA) is only valid when installed or used as instructed in the Installation Instructions.

You may need to and it would be cleaner to change the existing receptacle to a junction box and extend the circuit to the oven location.