Can I use the existing carpet underlay with plank flooring? We do not advise the use of carpet underlay as this is not suitable for plank flooring. It does not support the planks and would cause the boards to move, resulting in gaps in the floor.
Can I use existing carpet underlay beneath my new wood floor? No you cannot. Carpet underlay has too much give in it. If you fit your hardwood floor on top of carpet underlay the boards will move and will either squeak or come apart - or both.
In our last house we had a concrete floor. We a sheet of something under the wood floor that went down, it was like that very slightly foamy polythene stuff you sometimes get in packaging. It was very thin,maybe 6mm? And not very expensive, less than £2 per m2. The difference is that it is much less squishy than carpet underlay and you get minimal movement between the boards. I think you can even get a sound absorbing kind, but maybe that is more expensive.