I have not upgraded to 5.2 yet so cannot comment based on personal experience, but I have seen a related discussion with a few more details go by on the Lightroom Blog:
This was first mentioned in the thread here 'Seems V5.2 is final'
A plug in filter was mentioned 'Allfilters1.5' by JEllis (30 day trial then voluntary payment) which will find ALL images that had the heal/clone brush applied.
I used it on each month of this year 2013 rather than the whole year at once because its a bit overwhelming if there are many images.
Fortunately, I only updated this current year to V5.2 BUT there were many images with little clone brush applications, total of 1700 or so..
Initially, I clicked on each circle and returned the feather to its original number. The number is there at first view; it just isn't applied. This is a tedious exercise.
Eventually I decided the image could survive WITHOUT the sharp edged circle so I selected one, reset the clone brush and then synchronized the rest of the images for that month to have NO clone.
In these cases it was predominantly bokeh circles or basically insignificant things.
When it was power lines removed with overlapping circles I just redid that with the current painted clone rather than clicking on each one and reapplying the feather because I noticed that 5.2 has a little difficulty with the v4.4 overlap circles.
The upside is that this 'exercise' is a good, repetitve way to get familiar with the new style of clone brush. Click once firmly and its the old style one circle repair, Click and drag and its the paint brush which I still haven't managed to paint to get the edges <grin>
I'm not updating my past years 2007 to 2012 stored on an external with its own catalog to the 5.2 version since once was enough to do this 'brush' repair.
There's really no need yet to do any repair (unless the image is urgently needed for publishing)....Adobe have acknowledged that this is a bug and have stated that they are working on a fix which will correct the feathering back to what it was.
More discussion here:
It appears images with Spot Removal editing done in LR4 and earlier have Feather set to '0' and images edited with Spot Removal in LR5.0 are set to '75.'
The plugin mentioned for finding images with Spot Removal applied is AnyFilter:
There are also numerous other plugins that have the same capability to filter on Develop settings.