Perhaps we are missing something in the communication... but...
You say that there is no failure now... Unless you are planning a drastic remodel why would you need to move the post in the future??? Besides - the addition of this solid pad would NOT prevent you from doing anything you can do now as this modification does not change the manner of attachment of the post/cabintop connection...
Perhaps you are not confident of the strength/durability of the suggested repair?
Consider that the original cored construction has done quite well all these years...
Solid epoxy is much stronger... And it is as excellent material for compressive resistance...
Let's consider:
The top of your post has a metal bearing plate which is some 6" in diameter. (The mast base is 8" - but let's consider the smallest area as an example as it is the weak element in this scenario)...
This plate bears on the inside of the cabintop below the Balsa core (which is already solid FG and will remain. You'll only remove the balsa core and will replace the FG layer at top.)
The compressive strength of epoxy varies a bit depending on the product, type of product, and manner of application, but it will range between 5,000 and 10,000 lbs/sq-in. Glass makes it stronger, but let's ignore that...
Let's say that you use bad epoxy and your install is full of holes and poorly applied... so we'll use 5,000 lbs/sq-in.
The bearing strength under these circumstances for the solid epoxy pad will be as follows:
The Area of the 6" plate at top of post 28.26 sq-in x 5,000 lbs/Sq-in = 141,300 lbs
So you see - the pad is actually the support system's new strongest point - far from being the weakest... the mast will crumble, the compression post will become a pancake, and everything -including the keel - would be pushed out the bottom of the boat WAAAY before the bearing pad you build even starts to consider failing....
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