How do I replace rotten Subfloor under a supporting post
Im Not For sure I'm Understanding What your saying But any time I have to work on a Support, Here what i do ! 

For Instance I Just Replaced A 6x6 Post That Is under a roof Over Hang On A House!  I took A Car Jack And  Used A 4x4 I Put It Under The Beam That is sitting On the 6x6 Im Replacing. I Lifted The Jack Just enough To Take The weight Off The 6x6. 

I then Pulled It Out and Put the new one in! You Could Probably Use That! If your support Is Holding More then Just One spot youll need to support what ever it is supporting! Just take take the weight off the post change the subfloor under it and put the weight back on!!!

Hope that was help full    Good Luck