I do think that the change to 'mother' would be a logical one though, given what we've seen of Obi Wan and Padme in all six films.
In Sith, Obi Wan calls Anakin 'a very great threat', even going so far as to try and kill him. In A New Hope, Obi Wan says Vader killed Anakin, from a certain point of view, thus implying that he thinks that there is no good left in Vader, emphasised further in Jedi when Obi Wan tells Luke 'He's more machine now than man...twisted and evil'. Heck, from the get go Luke was being trained to eradicate Vader, to destroy the Sith, and return the Jedi. So, it's hard to imagine Obi Wan as ever thinking that Vader was redeemable.
Padme, on the other hand, actively tried to bring Anakin back from the Dark Side on Mustafar, and her last words before she died were 'There's still good in him', so Luke's mother did think along the same lines that Vader was not purely evil.
And having Vader mention for the first time Luke's mother in this scene could serve to highlight his gradual return to the Light Side, which I believe this part of Return of the Jedi seeks to do. Having embraced his lost son and now recalling his wife, the scene could effectively establish further humanity in Vader, building to his redemption and hinting that Vader will become a Jedi once more. The dialogue alteration here could perhaps strucutre Vader's return to grace more effectively, and create a more effective character arc then, making Vader saving his son and throwing Palpy down the shaft less sudden.
But if it were to be changed, it would have to be dealt with very cleverly. Vader wouldn't just recall his wife out of nowhere, and blatantly. So, instead of 'Your mother once thought as you do', it could maybe be inflected 'Your...mother...once thought as you do'. Then, possibly cut to a close up of Luke's face and cue Padme's theme playing very softly in the background. But I don't know if this is at all possible, as I haven't heard the audio source of James Earl Jones saying 'Mother' (if anyone could point me in the direction of it, I would be very interested to hear it).
However, is changing this line changing the original trilogy to fit in with the prequel trilogy? This is a dangerous path to take; surely the Prequels should be changed to fit in with the Originals, if anything.
Hmmmm...Very interesting...