Can i use any blender build with yafaray for any project or will it have major change that will unable to open the save file after.
There are some custom builds on graphicall.org. I think they are all windows, however. It does work, I have rendered some Yafaray scenes from Blender 2.5.
As far as open/save at a later date, I have no idea. Why would it break?
Do you mean you want to update an older file from 2.51 to 2.57?
There may be some minor differences in the look of the render, but there will be more, and more stable functionality. Many more parameters are animateable now.
The only problem you may have would be with Blender, as some things may not work the same way now due to the subsequent improvements that have been made.
The best thing to do is download a recent build from graphicall and try it.