Hi,

If you just change the oil and now have driven it, you are probably only low on fluid.
As little as a half a quart can result in improper shifts or more on a drop out of reverse not working.
Your transmission might be even be a “lock up” converter type in the 900 series.
I only have 240s so I have no experience in them. I have only heard of them and never drove one, that I known of, in any car.

What you thought was on the full mark has now stopped up inside the transmission in what was an emptied cavity.
The valve body is full of them let alone getting the torque converter chamber topped off.
Make sure you have driven the transmission about eight miles and while parked select all the gears while stopped. Do all that just before reading the dip stick for an accurate assessment.

On lots of automatics transmissions when there is a problem with going into the reverse gear it’s because of a low fluid issue.

It’s the first sign that tell mechanics to check it right off the bat.
Hopefully it want be a bunt! (:-)

Phil