Solr 7.4 index not updated after upgrading Solr to 8.11 - solr

Question

I am upgrading my Java program from Solr 7.4 to Solr 8.11.1 (same upgrade for SolrJ). It does not use SolrCloud.
I did not change the code base because the upgrade guide reads :
If you are not using Solr in SolrCloud mode (you use Standalone Mode instead), we expect you can upgrade to Solr 8 from any 7.x version without major issues.
However the old index is never updated (its contents does not change). I know it is recommended to reindex when upgrading to a new major version, and if I do so on a 2 document index I get the expected contents. But as it took weeks to process and index the actual data some years ago I'd rather avoid reindexing if it is possible!
I also tried to apply the upgrader tool with and without -delete-prior-commits and although it ran smoothly the old index did not get updated afterwards when used in my program.
My test index has 2 documents in it. During the index update in my program I do a deleteById(coreName,listOfDocIdsToBeRemoved) on a SolrClient which returns {responseHeader={status=0,QTime=1}} and just afterwards I do a commit(coreName) on the same SolrClient which returns {responseHeader={status=0,QTime=23}}.
Following the commit if I run mySolrClient.query(coreName,new SolrQuery("*:*")).getResults().getNumFound();, I still get 2 documents whereas it should be 0 (the expected result I get when doing a full reindex). So it looks like the old index is read only (although file permissions are the same compared to the full reindexed version of the index).
If I log to the Solr Admin dashboard, the core overview reads (the version keeps increasing as expected as the last modified field gets updated) :
Last Modified:
3 minutes ago
Num Docs:
2
Max Doc:
2
Heap Memory Usage:
2452
Deleted Docs:
0
Version:
56
Segment Count:
1
Current: V
Finally my question is : Can I still upgrade Solr from 7.4 to 8.11 and use the old index without fully reindexing or is it just impossible without fully reindexing ?
Please note : when doing a full reindex I use the old solrconfig.xml file in which I only update the luceneMatchVersion tag and I use the same managed-schema file.