Here's a couple that kind of match your description but I don't think they have a download option... They are pretty old though so you most likely have read these already.
Harry Potter and the Ancient Race (https://www.fanfiction.net/s/8416878/1/Harry-Potter-and-the-Ancient-Race) By Mione5 Once again by Kimberjingle (http://afallenangel.net/kimberjingle/kindex.html). This one does contain switching.
And on AO3 Corruption Series (http://archiveofourown.org/series/1798) By Beren
no subject
Harry Potter and the Ancient Race (https://www.fanfiction.net/s/8416878/1/Harry-Potter-and-the-Ancient-Race) By Mione5
Once again by Kimberjingle (http://afallenangel.net/kimberjingle/kindex.html). This one does contain switching.
And on AO3
Corruption Series (http://archiveofourown.org/series/1798) By Beren
Hopefully, they meet your criteria.