Lync 2013 client is deployed but user accounts are not migrated from OCS to Lync 2013 Server - how to open Lync meetings automatically in the Lync Web Plug-in

Question

We have in our enterprise the following scenario:
1 - Lync 2013 client is installed
2 - User accounts are not migrated to Lync 2013 Server, users are using Office Communicator as their main tool
3 - Users receive Lync 2013 meeting requests but when try to access them, Lync 2013 client launches and shows error. Users will need to open the browser and paste the URL to the address bar but this still open
4 - We cannot use the workaround of adding "?SL=1" to the Lync 2013 meeting URL as the user base is large and manual workaround is not accepted
5 - Question: is there any automated way, via egistry key or GPO setting, so that users temporarily (until their accounts are migrated to Lync 2013 server) can bypass Lync 2013 client completely and automatically open all Lync 2013 meetings
on the browser, using Lync Web Plug-in?

Answer 1

Thanks for the response,
First, I should have mentioned clearly that users have Office Communicator 2007 client and Lync 2013 client installed in their machines. Their accounts are not migrated yet to Lync 2013 server.
Second, we are using IE9 and IE10. The issue is that users CAN join Lync 2013 meetings with their browsers but have to paste the URL manually to browser and add "?SL=1" otherwise, if they just click at the "Join Online Meeting" or "Join
Lync Meeting" URL it launches Lync 2013 client which shows error because is not configured yet, as they are using OCS client and migrating slowly to Lync 2013 server.
Is there a Group Policy setting or a registry key from Microsoft that can be turned on to these users machines and make will all Lync meeting requests to be opened in IE browser instead of Lync 2013 client. We need a way to ignore
Lync 2013 client until user accounts are migrated to Lync 2013 Server. Manually typing URLs is not an option in a big organization, can't explain thousands of users of different levels what to do.
We are regretting the decision not to separate Lync 2013 from Office 2013 package we deployed recently. If Lync 2013 is uninstalled then all Lync meeting requests are opened in browser without an issue.