Without sounding stupid – it depends on the changes.
The biggest issue when upgrading is not just the ‘look and feel’ of the theme, but all the functions that are used by the theme. For example if your theme uses add_option() it may not work, later versions use update_option() instead (as it creates the option if it does not already exist).
As we do WP work I personally always download a new WP version onto my local PC, install it and run my themes on it using the WP test data before updating, probably a luxury for most users, but it is worth the extra time for us.
Failing that ensure you have saved a complete backup of all files and the database before upgrading, as I would say there would be a fair chance of a bespoke theme failing on upgrade.
Good point.
My theme doesn’t use particular functions (functions.php) but 3 homebuilt functions to separate and count comments and trackbacks.
Other than that, I usually compress the biggest js and css WP files just to save some bytes without using any 3rd party plugin.
Sounds as if you might be OK, though don’t take my word on that! The biggest problems we get on upgrades are the system functions such as has_cap(). has_cap() sits in a file somewhere in the wp-includes directory. Older versions need a Integer value, newer versions need a text value with a specific role.
If you set your wp-config value of WP_DEBUG to true you will see the errors/warnings on screen. Not very pretty at the best of times 😉
At the end of the day it is a value judgement on your part, do you need an update now and if so can you set aside the time to deal with it, and possibly problems resulting from it.
