If it was me, i'd put two or three holes in the bottom of the pond with a fairly large drill bit. This will allow the water to drain out so the bog garden is not sitting in permanant water. Bog plants like it damp but not submerged....those plants are classed as marginal plants.

A good top soil mixed with a copious amount of good organic matter such as well rotted leaf mould. Bog plants love rich soils.

You will have to choose your plants carefully as may are fairly large and some can be very invasive.

As an alternative...somehow break up the bottom of the pond and half fill with pea beech or gravel. Add good top soil and turn the pond into a rock garden or alpine scree with small spreading saxafrages, dwarf bulbs and other lovely alpines.

Just a thought.

Or if you still like the pond you could keep it by using a butyl pond liner over the original concrete. they you could have a few baskets at the shallow end for some bog plants and have the best of both worlds.

Thank you, some things to think about. I quite like your alternative idea Andy.