My experience with codex upgrades (although I'm not a Tau player) comes down to a few generalizations:
1) Wargear tends to have the biggest changes. The latest round of 'dexes has reduced the variety of war gear available to any given unit. Particularly affected in this regard are squad leaders IMO.
2) Special rules to tailor armies have largely gone by the wayside in favor of introducing more characters who give you a more limited ability to tailor than say Imperial Guard doctrines or old Space Marine chapter customizaiton. This can cause some pain if you've use these special rules and have a WYSIWYG army.
3) Codices rarely take away a unit. The main impact to your army list at an unit level is either changes in abilities or the occasional switching of a unit type from elite to fast or heavy and vice versa.
Short of it, if you buy the battalion box and keep your list fairly vanilla, you shouldn't run into many, if any, problems with new dex in terms of being able to use your existing models. Some of the more extensive wargear options may be trimmed, but usually it's to remove stuff hardly anyone used anyway or to make it standard when almost everyone took that option anyway.