The main reason to convert high heels to wedges is for more comfort.
Can you make heels into wedges.
Just cut it off with a small saw when it hardens to the level of the soles heels.
You can buy new tips for the heels though to stop them being slippy.
Most shoe shops sell them for about 50p.
You couldn t do the foam thing with fabric because it would bulge out at the sides when it set.
As an alternative you might want to convert high heels to wedges for more comfort and style for some kinds of wedding then get some tips and top choices here.
That way you would have the rigidity of a full wedge heel without the weight as if it were full wood plastic or whatever the rest of the sole is made of.