I work with thousands of part numbers. Some of the referenced items are now unavailable, and a replacement has been found. 

In communicating to my users, do I say 
"x `supercedes` to y" or "x `supersedes` to y" ?

And in discussing the practice, do I term it `supercession` or `supersession` ?

Should I write 'x has been `superceded` to y' or 'x has been `superseded` to y' ?

And then, is it different when referring to it from after the fact, eg: 'y `superceded` x in January' ?

I can't find any answers on this site that address this directly. I have googled it and found conflicting renditions so am no further ahead!