Modified a bit. This should show for anyone who has GnuCash installed.

The 2.7.3 ebuild I have in my overlay does have a postinst note about
this as well, but I think this is important enough to tell them as soon
as possible and on systems that may never have had GnuCash installed but
will be working with files/databases that are made by GnuCash 2.6.

Title: GnuCash 2.7+ Breaking Change
Author: Aaron W. Swenson <titanofold@gentoo.org>
Posted: 2018-01-10
Revision: 1
News-Item-Format: 2.0
Display-If-Installed: app-office/gnucash

Along with changes to updates to use modern libraries, GnuCash 2.7+ has
changed the schema [1] it uses for both databases and files. GnuCash
will automatically modify the file or database in place upon open.

Therefore, it is imperative that you back up any files or databases
before using GnuCash 2.7 in case you run into an issue and want or need
to revert back to 2.6.

Instructions for backing up are as follows:

For XML (plain files):
$ cp /path/to/file.gnucash /path/to/file.gnucash.bak

For MySQL:
$ mysqldump gnucash_db | mysql gnucash_db_bak

For PostgreSQL:
$ createdb -U dbadmin -T gnucash_db -O gnucash_user gnucash_db_bak

For SQLite:
$ cp /path/to/gnucash/sqlite.file.gnucash /path/to/gnucash/sqlite.file.gnucash.bak

[1] https://github.com/Gnucash/gnucash/releases/tag/2.7.0a