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.
