Being the good friend I am, I was able to give my friends 20ga. 870 a new home since its just sat around his house since he left for college.  I dont really have a need for another 20 gauge,(but the price was right) is there any chance that it could be converted to 12 ga by going through the parts list and finding all the differences? Or is the receiver itself different? Thanks for any info.

first it's a Gauge.  not a caliber.

i don't know if you can do this but i think you would be better off selling it and buying a 12 gauge.  you would have a brand new gun and won't have a lot of parts left over you could not get rid of.

The receivers are different.