[renehaentjens]

I did a first try on my test server, proceeding as follows:

# - saved a copy of includes/connect.php, config.php
# - made the PhpDig 1.8.3 directory inaccessible
# - restored the 1.8.6 backup to a new directory .../phpdig-1.8.6
# - replaced phpdig-1.8.6/sql/update_db.sql by the merge of update_db_to_1_8_4.sql, update_db_to_1_8_5.sql, and update_db_to_1_8_6.sql, as explained in a reply above
# - ran phpdig-1.8.6/admin/install.php, selecting "Update existing database", and providing the parameters from the old connect.php
# - adapted config.php (cfr. my old config.php)
# - re-applied customizations

Seems to work... no data lost... I'll do more tests and report here when needed.