Whatever you do, all of the existing post hole must be removed. Face it,  
you are going to do some damage to the foam in that area. You can't make  
scrambled eggs without cracking a few. Drill it and dig it out as Stan  
suggested. Clean out the area making sure the existing foam is dry and just  prior 
to installing the new one, fill the damaged areas with "Great Stuff" foam  
and then install the new base. Remember, this stuff expands a lot, so try to  
judge just how much you will need. Try putting some on a scrap piece of  
cardboard or plywood and experiment with how much you think you will need.  
Any excess can be easily removed, but once hard it will have to be cut  out.
 
Rummy
 
 
In a message dated 8/18/2009 12:34:20 P.M. Eastern Daylight Time,  
bgreenwald at optonline.net writes:

Todd,Thanks for the suggestion - your picture is accurate except  that the 
flange is completely gone - now that I have had at it.  I will  be able to 
get a better sense of the condition of the foam once I get the  remainder of 
the  post holder out but from what I can feel with my  fingers seems pretty 
firm - no doubt water has visited though, even with the  various efforts I 
have made to keep the hole covered.  In any event, I  think a saw either 
around the outside or from the inside out may be the only  choice.  It has 
occurred to me that since the post holder in the floor  seems firm and in good 
shape maybe all I need do is seal the bottom - that  approach would certainly 
cause the least amount of intrusion/potential damage  even if the result 
wouldn't lok as finished as a new piece  altogether.