Your question is a bit tricky because it depends on which types of block you speak of.
After the merge, the beacon blocks will encapsulate all the info and thus can be considered as "the main block". Their header will look like this:
But your question seems to be focused on the execution layer type of block. And even if the block header is as I've shown above, most of the info previously in the header of execution layer will remain available. This picture gives a good overview of what will change:
The change indicated on the picture isn't the only one, as the website where this image comes from states:
Post-merge, several fields previously contained in proof of work block headers become unused as they are irrelevant to proof of stake. In order to minimize disruption to tooling and infrastructure, these fields are set to 0.
And the changes are the following:
Because proof of stake does not naturally produce ommers (a.k.a. uncle blocks) like proof of work, the list of these in each block (
ommers) will be empty, and the hash of this list (
ommersHash) will become the RLP-encoded hash of an empty list. Similarly, because
nonce are features of proof of work, these will be set to
0, while respecting their byte-size values.
mixHash, another mining-related field, won’t be set to 0 but will instead contain the beacon chain’s RANDAO value.