I noticed a similar problem on one of my quilts. I had hand quilted the majority of the quilt. I had a star block that the points were facing the wrong direction. I was able to pull the block out, fix it, reinstall and go on my merry way.
I'm not overly familiar with interfacings or stabilizers but I think I would rip it apart and flip the block over. Tedious process to be sure but since it is a memory-type quilt, I would feel compelled to fix it and 'make it right'.
Good luck!