TOE's work counter-intuitively in the editor.
The end date for the previous TOE is the date the specified "succession" TOE (specified in the TOE location, not the unit location) becomes available as an upgrade.
This is mixed in with some in game filters that give the player a great deal of control.
If you give a unit a quite unique mix of devices, but connect it to a TOE, that ends at Date X, allowing an upgrade to another TOE, then if the player-in-game never puts it on (X) rest/(Y)upgrade within (Z)range of a command HQ it will never graduate to the next TOE.
On the other hand, the player can hold it where it is (even allowing upgrading of individual devices) as long as XYZ is never satisfied.
Eventually the player can allow XYZ conditions to get, say, a late war upgrade, but doesn't have to.
"You may find that having is not so nearly pleasing a thing as wanting. It is not logical, but it is often true."
- Cdr Spock