Remember that 2008R2 and below are all now out of mainstream support and extended support.
SQL 2012 and SQL 2014 are now out of mainstream support.
These legacy versions of the engine shouldn't really be supported any more I appreciate that your business is to make money but having to keep supporting legacy software is going to have a major overhead to your development processes.
You're going to have to draw a line somewhere, I would say do it now and in the next release of the software you say you no longer support 2008R2 or below and then start to phase out 2012 and 2014.
But back to the issue at hand, if you want to have the 2008R2 or below syntax you have to run it on that particular version of the software. Once you upgrade the SQL version you also upgrade the database, compatibility level is for internal use only it is not for user use.