this is my experience to upgrade since 4.9.5 to 5.2.3 and all is ok for me

make sure, you are running on SQL Server 2005 or 2008 (and your database is not in SQL server 2000 compatibility mode)
and .NET 3.5 SP1 .
compatibility mode on SQL 2005: select database = > property => option => level compatibility

to upgrade:

* backup all files and database (dotnetnuke + master + model + msdb), you can detached the dotnetnuke database and save the DotNetNuke.mdf and DotNetNuke_log.ldf
attache befor upgrade.
* Backup web.config machineKey validationKey="" and decryptionKey="" to put on the new web.config
* stop the website or at least turn off AutoUpgrade in web.config (edit with a text editor)
* backup all files /Dotnetnuke folder website
* download upgrade package and unzip files hierarchically into DNN installation folder
* verify in web.release if [ appsettings ] is present: : before

* web.config is ok now
* start web site
* if autoupgrade turned off, browse to /install/install.aspx?mode=upgrade to trigger upgrade
* review the upgrade log on the screen
* if it went well, go to Host :: Extensions to upgrade modules. HTML should be upgraded automatically.
* upgrade form and List is successor of UserDefinedTable and will keep all data.
* New versions of Forums, Reports and other modules are included as well.

when i have upgrade i must install form and List and this module replace UserDefinedTable.
if all run correcly, stop the website and backup all files again ( databases and website folder)

start web site, it is possible that you have "message" in website for blog module and event module i have just select edit module and save to run correcly the modules.
the modules installed before upgrade are the latest version supported by 4.9.5
This is a basis for reflection and not a How to, if that can help someone before embarking on its own update
always perform a full backup before doing anything