Transmission

Once a transmission begins to slip there usually no fixing it especially from changing the fluid. In fact the old clutch material suspended in the old fluid might have kept reverse working as long as it did. Either way you need a new transmission or your current one rebuilt, the clutches are most likely worn out.

You will need to have all the codes cleared and retrain the trans shifting. Its as simple as going through the gears. Make sure all your fluid levels are where they need to be first then do the trans learning procedure.

Have it read by a scanner to see if it shows any codes. Im betting it probably threw a PO736 "Reverse Incorrect Ratio" Its as simple as resetting the computer or letting the computer relearn the correct gear positions. There are several procedures that can be done that are very simple to do.