Upgrade Bluetooth to 4.0
Can the Airport Extreme Card of the iMac 27" EMC 2374 be upgraded to have Bluetooth 4.0 to take advantage of the OSX Yosemite hand-off features?
It is possible, but not practicle. It would be a lot of work to get to it and replace it. It would also be a task to find the right replacement card. And, you'd still have a lot of "marginal" issues with trying to run that OS on a machine this age.
AFAIK Bluetooth 4 is backward compatible. It's main advantage is for use in mobile devices.
Unless you have a current peripheral that will not connect to your machine, I wouldn't think you'd get much benefit from the upgrade.
If you do have such a problem I think selling your current machine and upgrading to a new(er) one that can run Mavericks would be a better solution.
The Mac mini and MacBook Air were both updated with Bluetooth 4.0 support in 2011.
The MacBook Pro and iMac added it a year later in 2012.
The Mac Pro model debuted in December 2013.
If this answer is acceptable please remember to return and mark it.