Some people who have bought an ebook reader of some type, after less than a year, have changed them for an other make that uses an other format. It is not logical to reformat an entire collection at once.(I have several thousand ebooks)
I find myself often tempted to buy an other ebook reader. Does it use the formats my ebooks are in? Which ones will I read next?
Now I keep less than 20 ebooks on the device, to me it's 6 months of reading! Will my device still work in 6 months? Will I buy an other device? If so, will I keep the present one or trade it away?
Carpe Diem.
A better way of using that formating time would be to clean your rooms.

Or read.