One option would be to look at the messages.log files since you put the upgraded version into production and extract our the usernames of those who have connected since that time (assuming you have all those log files). Not trivial but the exdtraction process can be automated through parsing scripts.
Details of the messages.log file format can be found at Agentry Messages Log Definition - SAP Mobility - SCN Wiki
The other option would be to see if there are any SAP logs that might track the last login of your users compared to when the new version was put into production.
--Bill