ᐅ Widening concrete fence posts

Hello,
I have old fence posts that are set in concrete in the ground. They are made of concrete. I now have new fence panels that are narrower than the old ones, and I would like to widen the existing fence posts. My idea was to create a formwork and simply apply concrete with a thickness of 3–7 cm (1.2–2.8 inches) using the formwork.
The existing posts are painted with wall paint. My concern is that the new concrete might not bond properly with the old one. How can this be prevented? Or does anyone perhaps have another idea?
I look forward to your replies.
Best regards
Michael

3 cm (1 inch) of concrete is insufficient; it will crack and fall off due to frost. The coating also prevents bonding from forming, so only a high-pressure cleaner can help. Why don’t you reinforce it with an appropriate squared timber instead?

Thanks for the tip. I can remove the paint with a pressure washer. The fence panels are made of metal, and wooden extensions don’t look very good. Since I also need different widths (the posts are sometimes a bit further apart, so a thickening between 3 and 7 cm (1 and 3 inches) is necessary), wood wouldn’t look great and would weather quickly here. (The wooden fence only lasted about 6 years despite maintenance.)

Now I have found this:
doitBau construction board XPS
Maybe this could be an option if you cut it to size, fold it open, and then plaster it all over. Does anyone know if dowels hold in it and what weight it can support?

That might work somehow, but it won’t improve the strength. Besides, these Wedi panels aren’t exactly cheap.
Why not just knock them off and install new wooden posts next to them? It’s easier, faster, and cheaper—or you could use lightweight metal posts.
An approach like the one you described will involve a lot of effort, time, and cost, and in the end, it still won’t look good.
If you upload a photo, it might be possible to come up with better ideas for a solution that fits a garden setting.
With concrete, those would basically turn into proper bridge piers...

The posts are definitely going to stay and will be planted with ivy later on. So if it doesn’t look perfect right away, that’s less of an issue. But you can see in the photo that the existing fence panels have quite a lot of space between them, and the new metal panels need to fit flush.