Drop-in in YYZ is definitely fine. HOWEVER, unless one is a US citizen, they will take away your NEXUS card and mail you a new one. The Global Entry to US will be active immediately as it is lined to your passport, so it is good at the airport entry to US. On the way back to Canada or at a land entry to US, one cannot use NEXUS until the new card arrives. [I got this information in 2015 from the NEXUS office at Whirlpool Bridge].