Gregy, try these steps and see if your models show in older versions of maya.

1 - save your scene in maya6.5 as ma
2- Open the ma file in Notepad
3- change all 6.5 to 6.0
4- save it ... file > save
5-Open your scene in maya6



Here is an example

1 - save your scene in maya6.5 as ma
2- Open the ma file in Notepad
3- change all 6.5 to 4.5
4- save it ... file > save
5-Open your scene in maya 4.5

I'm not absolutly shure it's going to work but try it anyway.