Once selected, is the Tumor Familiar shape permanent?

Question

The Tumor Familiar discovery from the Alchemist base class states:

Benefit: The alchemist creates a Diminutive or Tiny tumor on his body, usually on his back or stomach. As a standard action, the alchemist can have the tumor detach itself from his body as a separate creature vaguely resembling a kind of animal suitable for a familiar (bat, cat, and so on) and move about as if it were an independent creature. The tumor can reattach itself to the alchemist as a standard action. The tumor has all the abilities of the animal it resembles (for example, a batlike tumor can fly) and familiar abilities based on the alchemist’s caster level (though some familiar abilities may be useless to an alchemist). The tumor acts as the alchemist’s familiar whether attached or separated (providing a skill bonus, the Alertness feat, and so on). When attached to the alchemist, the tumor has fast healing 5. An alchemist’s extracts and mutagens are considered spells for the purposes of familiar abilities like share spells and deliver touch spells. If a tumor familiar is lost or dies, it can be replaced 1 week later through a specialized procedure that costs 200 gp per alchemist level. The ritual takes 8 hours to complete.

What is unclear here is, does the Alchemist choose the form his tumor resemble when he makes the discovery, or does he choose a shape every time the tumor detaches itself? The wording would suggest that the creature is chosen every time the tumor detach itself, but then it states that the tumor also acts as a familiar when attached to the alchemist, giving it the relevant bonuses. This suggests to me that only one shape might be chosen and that is the shape all the time, every time.

Answer 1

As written it seems the shape of the tumor can be changed each time it is detached from the alchemist's body:

the alchemist can have the tumor detach itself from his body as a separate creature vaguely resembling a kind of animal suitable for a familiar

There is no mention of the first time the tumor is detached being different from the other times, so each time it detaches it gets a shape.

However it seems likely to me that what was meant here was that you had to choose the shape of the tumor the first time it detaches. I think this mostly for a question of balance as making you able to change the tumor's shape at-will makes you able to get +3 to any skill when needed, among other strict benefits compared to a regular familiar.

Answer 2

The tumor acts as the alchemist’s familiar whether attached or separated

Tumor Familiar says it works as the alchemist's Familiar, so whenever you create a new tumor familiar, you can pick their form (bat, cat, and so on). This is also true when you are replacing your familiar.

Finding a Replacement

In some cases, replacing an animal companion or familiar can be as easy as purchasing an animal of the desired type and declaring it your new companion.

Attuning a familiar to its new master requires a ritual. Choosing an animal companion requires 24 hours of prayer. The ceremony can also be used to attract and bond with an animal appropriate to the local environment. However, you might want to wait for the campaign to present an appropriate companion, such as an animal you rescue from a cruel enemy that you tame with the ritual or ceremony.

In terms of game mechanics, there is no difference between any of these options, and you should work with the GM to find a replacement method that is appropriate to the campaign.