If you're thinking that upgrading to Magento 2.3 will be a piece of cake - make sure you do the prep work first!
With every Magento release they post a rather nice user guide with detailed instructions on how to upgrade.
Following these instructions should get you almost there - the problem you may find though is with your third-party Magento 2 extensions.
While upgrading throughout the 2.2.X process, you might not have had any issues with your extensions, this is because they all use a default composer package version for the Magento framework of 101.X which typically includes any 2.2.X version, therefore composer doesn't complain too much during these minor updates and most extensions cover these versions in their own composer dependencies.
With Magento 2.3 however the version numbers for the framework packages are upped to 102.x which none of your previous extensions are probably going to be able to work with without updating them first!
In short, make sure you go through your list of extensions first, to make sure that
- A) They're up to date and working with Magento 2.3 and;
- B) They're set to work with PHP 7.2
As of this writing, Magento 2.3 was released last night, I've attempted a local upgrade this morning and have come across a number of well-known extension compatibility issues. However, in true Magento community spirit, the turn-around of the fixes has been extraordinary!
Granted, the only change these developers needed to do was to allow composer compatibility with the 102.X framework versions of Magento and PHP 7.2.
Currently, another popular package - the MagePlaza SMTP extension has a more serious issue, where the Zend_Application_Resource_Mail has been dropped by Magento. So unfortunately in its current form this creates an error as the class no longer exists. A little bit of coding and re-working will be required in order for this extension to be compatible with Magento 2.3.
So if you're having trouble upgrading your best bet is to get in touch with your friendly extension developer to see if there's a timescale as to when they believe their extension will be Magento 2.3 compatible, most of the time they'll either be already doing it or will provide you with an ETA.