End user experience
I'm sure WA staff will answer this - but this is my experience as an end user.
Once you have installed the MYSQLi extension you will the be able to create MQSQLi connections, recordsets etc as shown in the tutorials.
Existing pages will not be 'converted' - but - you are able to move forward.
For example: if you have an old page using an existing (non MYSQLi) connection to a database, you can build a new page but this time specifying a MYSLQi conection to the same database.
You can then populate the new page with MYSQLi recordsets and behaviours and when it is working correctly, replace the old page.
I have done this a page at a time on an existing web site,
Of course all of the old (non MYLSQLi) extensions will still be there to use if you wish.