Technically, No, but it depends what you are trying to do and willing to lose.
A "downgrade" is really a image with the old operating system and restore the backup file. If you just upgraded, then you are not going to lose anything by putting the old OS back on it and restoring from the backup file you made. I went through this when 5.0 was messed up when it first came out. I was particularly mad because we waited a few business days on support thinking they might fix it or "downgrade". Come to find out downgrade is re-image and restore. At least that was my experience.
Thank you Chris. We have directed by Fortinet to downgrade from 5.5 to 4.3. We are experiencing multiple issues - HA fail-over not working and chronic time adjust issue on Primary. Were your issues similar?