This was probably addressed at some point in the past, but I was having trouble finding previous discussions about it.
Certain familiars require their masters to be of a specific alignment in order to obtain them. For example:
A chaotic good 7th-level spellcaster can gain a lyrakien as a familiar if she has the Improved Familiar feat.
What if, later in the campaign, the spellcaster were to have a change in alignment? For example, the CG spellcaster decides that being a rabble-rouser is getting them into too much trouble and they start to become NG?
Would the spellcaster retain the familiar because they were CG at the time they gained it, or would something else happen?
The rules for feats say if you no longer have the prerequisite for the feat you do not gain the benefit of the feat. While the feat itself does not have an alignment restriction your choice of familiar does. That would mean that although you still have the feat you lose access to the familiar, but can gain another type of familiar that requires improved familiar.