Web farm is not really supported, so you need to make sure extension you are using still works with newer version.
Version 2.0 (2010)
Migrate to ASP.NET 3.5/IIS-7
Multiple blogs on single installation
jQuery/AJAX out of the box
Better user and role management
Redesigned admin panel
Refactoring for better extensibility
Version 2.5 (June 2011)
Upgraded to .NET 4.0
Multiple Blogs in Single Installation
Razor Theme support (Razor theme included, "Garland-Revisited")
Converted Admin pages to Razor (Dashboard, Extensions, Themes)
Install Themes from the BlogEngine.NET Gallery while in the BlogEngine.NET control panel.
New language resource files for Estonian and Polish, and other translations updated.
Upgraded to jQuery 1.5.2.
Upgraded to latest version of tinyMCE WYSIWYG editor, v188.8.131.52.
Numerous fixes and improvements.
Version 2.6 (May 2012)
Complete gallery integration (handle all packages from admin UI)
Extend functionality of multiple blogs feature (site aggregation)
Built-in File Manager with Image Tools
Version 2.7 (November 2012)
Added option to remove .aspx file extensions in URLs
Added option to turn JS and CSS optimization on/off
Improvements to user profile
Version 2.8 (April 2013)
New standard theme
Improvements to site aggregation
SQL Server provider works with Azure out of the box