If you're of a mechanical nature you might consider checking something; In my younger days I used to rebuild my own auto transmissions, on the C4 the big adjustment bolt sticking out the side w locknut (I'v seen something very similar on the 5R55) was for the low / reverse band (clutch) I once got one that had no reverse and wouldn't "hold back" in Low (1st gear position) turned out due to wear the band had loosened enough for the linkage to fall out of place. I took off the oil pan and maybe the valve body, I think, (it's been a while) loosened the adjustment and put everything back in place, readjusted and it worked fine.
I'v never worked on the 5R55 trans but if you're replacing it anyway you won't be out anything by trying...