Familiars and Animal Companions

Familiars:

The feat Pack Flanking can use a familiar rather than an animal companion, provided you have a familiar who can threaten squares (such as a goat). 

You can use one of your own feats to grant a feat to your familiar instead (to allow your familiar to gain additional feats). 

You may take Leadership to turn your familiar into a Familiar Companion. It would retain all it's familiar abilities, and count as both your familiar and your cohort. It would gain class levels (though it retains it's animal HD, minimum 1 animal HD). It does not gain increased stats from this (outside of it's familiar intelligence boost) though it can gain bonus stats from high HD. 

Animal Companions:

You may take Leadership on your animal companion to take it as an Awakened Companion. It retains all it's animal companion benefits, as well as the increased mental stats from awakened. It could gain class levels. 

Alternately, you can take Leadership on an awakened animal to make it just a cohort and not part of the animal companion class feature. You would be able to have another animal companion normally. In this case, the animal companion loses the special companion features (link, devotion) but keeps any additional HD or physical increases it obtained so far. (It does not gain others). It can continue to level up as an animal, or it could gain class levels instead.

Combining Animal Companions and Familiars
If you have both class features, you can have your animal companion also be your familiar, and gain the benefits of both on one creature. Instead of using your HD as your familiar's HD per the familiar benefit, you'd use his actual HD from the animal companion table. As he levels up in HD as an animal companion and gains increased intelligence as a familiar, he may have more skill points.