If you replace your network card or upgrade to a different version of
dhcpcd you might not be able to obtain the same old IP address from
DHCP server. This is because the DHCP server identifies clients by
ClientID DHCP option which by default is MAC address of the network
card. The work around is to use -I ClientID  option with some
unique "ClientID" string.
Also, upgrading to a different version of dhcpcd invalidates *.cache
file where dhcpcd stores IP address which it tries to renew on restart.