Big packages like ECB will be enhanced and developed continuously so sometimes a new version must be released. Such packages offer in general a lot of customizable options so probably some of these options change the type or are renamed because the old type and/or name of the option makes no sense in the new release.
Especially options which have changed the type of their value are now a problem for the user which want to upgrade to the latest ECB-version: If the user has saved a certain value for option X in its file .emacs but the type of this saved value doesn't match the new defined type in the defcustom-form after an ECB-upgrade then there can occur serious problems like ECB can not be started anymore or even Emacs can not be started without errors.
Until now there was only one way to fix these problems: The user must manually edit his file .emacs and remove all entries for options which have now another type. After this and after restarting Emacs the new default-values of the type-changed options in the new ECB-release are active and the user can go on using Emacs and ECB. But this approach to fix the incompatible-option-problem has two serious drawbacks:
OK, this is one half of the option-upgrade-problem but a new ECB-release can also rename a option from name X to name Y because the new name Y makes much more sense and/or is more mnemonic. If only the name has changed but not the type this is not a serious problem like above but also annoying because the customized value of the old-option X takes no effect in the new release but instead the default-value of the new-option Y is now active. But nevertheless this problem has the drawback number 2 (see above).
The last category of upgrade-problems is a renamed option which has also changed its type.
ECB has a solution for all these problems:
ecb-upgradable-option-alistwhich allows the ECB-maintainers to define special transformings for renamed options so even the value of an old-option X can be savely transformed to the new-option Y and the old setting is not lost.
All these checks and transformings are done at beginning of activating
ECB - if the option
ecb-auto-compatibility-check is not nil. If
ECB has recognized incompatible or renamed options it does its
upgrading/reseting-job so all ECB-options have correct types so ECB
can start correct. After ECB is started it displays a list of all
upgraded or reseted option with their old and new values.