I have not used Select2, but I do use Jonathan Tang's combobox plug-in and have found it easy to work with.
http://jonathan.tang.name/code/jquery_comboboxTang's also uses options to invoke and configure features. His plug-in may not have all the bells and whistles of Select2, but it is a true combobox.
The jQuery Combobox is not a true combobox and it is extremely complex code, with no options for alternate configuration. I don't recommend it unless it satisfies your needs "off the shelf". Its just my simple - don't waste your time - judgement.
Regarding upgrades - most of us like them. If you don't, you have the choice to stay on the 1.8.3 jQuery Core release. It will be around a long time. You won't get any new capabilites and HTML5 is only partially supported, but it is a choice.
I'm no guru, but I found the Migration Upgrade guide to be organized and clear. It says things like - if you have the function x(), replace it everywhere with the function y(), and says that over and over for every case. I just did a search on my code, and did the replacements. It took a couple hours with testing - just to be sure. The Console log messages helped with a couple that I somehow missed.