Q: I am about to purchase an iMac 27" Retina. How can i recover alla mu Vindows Office files tored in an external WD by book Essential and put them into the OS X Yosemite enviromnent ?Tnx for yr help

Tthis article provides a good overview How to transfer data from a PC to a Mac - Apple Support

There are other, more specific, articles available about migrating iTunes, etc, so perform a search Of Apple Support articles.

There's no switching necessary.

Simply plug the external drive into the iMac and use Finder to browse through it for the files. OSx is perfectly capable of reading Windows formatted drives.

You can then install either Microsoft Office For Mac, or Apple's Pages to open the files directly. No conversion necessary.

Connect the drive and copy via drag and drop.  Easy.