The problem is not to open the iMac, and yes, I think there are no problems with an SSD instead of the built-in HDD, according to some experience on the web it will take about 10 minutes to open the iMac, take away the HDD, replace it with an SSD, and make it work.
The real "problem", is to replace the glue stick to close the iMac properly....
I'm going to buy this iMac, and I'm thinking about the "adventure" to replace the HDD with the SSD (like I did on my old Early 07 MBP... (and no, there are no fan spinning crazily or other problem caused by trim or not, and however there's an app called trim enabler...)... But I'm really "scared" about this thing of the glue sticks to replace the original glue...