WARNING: upgrading to MS Office 365 may change document formatting

I've been a long-time user of MS Word (and Office) 2010.  In order to share files with Miss D. and have access to other useful features, I upgraded today to a family subscription to MS Office 365, the online version of MS Office, with software available locally and on the Web for up to 5 PC's.

All went well until I loaded the manuscript of my latest book.  All of a sudden some of my paragraph styles didn't work as they were supposed to, despite the document format (.DOCX) remaining the same.  As soon as I saved the document in MS Word 2013 (as used by Office 365), things became difficult.  However, when I re-saved the document in Word 1997-2003 format (.DOC), everything reverted to normal once more.

I have no idea what this means, but I thought I'd warn my readers:  if you upgrade to the latest version of MS Word, be aware that formatting problems may result, and you may have to save older documents in an older format rather than allow the program to save them in a newer version (and convert them in the process).  It does strange things.  I have no idea whether it'll still do that if I start a document 'native' in the new version, but I'll find out soon enough.

Peter