To be done in a development environment.

Choose a LimeSurvey 6.x version with several thousand questionnaires.

Install a LimeSurvey 7 beta version.

Modify the config.ini file of version 7 to point to the database of version 6.
Perform the update:
Via the web interface
After failure, repeat the update via the command line using the command:
php /limesurvey7/application/commands/console.php

The following errors may appear:

• PHP Error[2]: Undefined array key 0

• TypError: Unsupported operand types: sting - int in /limesurvey7/application/helpers/common_helper.php:1471

In my case, the second error occurred after I deleted 1,500 questionnaires.

It would be good to be able to create a log file to record these errors for analysis (and then disable this file) during an update.

forum link :
https://forums.limesurvey.org/forum/installation-a-update-issues/159180-3%20036-limesurvey-7-updating-an-existing-database-failed#274363