I already did some research on my question (see below), and it's as good as a 'done deal' but I would still like to put forward my problem to this knowledgeable community.
Short version of the issue: in partman (the disk partitioner of the Debian installer) the passphrase of a previously dm-crypt/LUKS encrypted volume was changed (or added) by mistake. The data on this volume was not flagged for removal. I cancelled the installation after that point. Later after manually 'decrypting' this volume it was found that only the ‘new’ password could decrypt the volume, but data could not be read (i.e. filesystem and files were not found)...
I was wondering if after changing back to the old passphrase I would be able to properly decrypt the volume's contents.
Previous research: the above question was submitted to the debian-boot mailing list, and there I received the following (very clear) answer:
I don't think the data will be recoverable unless you have a backup of the LUKS header. The way LUKS works is that data is not encrypted with a passphrase directly but with a key that is encrypted to a set of passphrases. If you worked purely through the installer's UI you will have overwritten your LUKS header and hence will be unable to decrypt the data ever again because the key material is lost. The position of the LUKS header on disk is always in the same place.
Data erase is really just about overwriting the existing data with zeros, which I understand is pretty confusing. Technically the data is already erased by the fact that the header is overwritten but some people want to be sure and write random data (or in the case of non-encrypted disks zeros) to the disk before deploying the system into production.
Alas, I do not have a backup of the LUKS header of this volume. As I said above, the intention was only to mount the previously encrypted volume, and not to change anything (so, to my regret, I didn't take the proper precautions).
The question: Is there any way to (re)generate the original LUKS header using the (known) original password against which this volume was encrypted, or is this data permanently lost?
Thank you for your consideration and your time.