I did this over the winter and was able to get all records for one horse and records for one horse as early as 1998 (though 98-99 weren't totally accurate), although I started showing him in 1992. When the change happened from AHSA -> USAE -> USEF there were apparently changes with how points were recorded/maintained in the database. I'd have to find the emails to tell you exactly what she said.
I simply emailed firstname.lastname@example.org with my USEF number and horse names/info and received a very nice email back from a CS rep that helped me throughout the process, even though I'm not currently a USEF member.
I was able to pull results from an old lease - I had her USEF number, which I pulled off of the USEF website. She is deceased, but she was lifetime registered and I knew the owner so her number was still there. It's worth a shot!