Questions about advancing druid's animal companion

[ayvango]

So, I should add extra HD for a companion

1. Would it change its size?
2. What feats could I select?
3. What skills considered cross-class and what are class skills for animals/elementals?

And what should I do if I released current companion from service and obtained new one?

4. Would it immediately get upgrade after coming to service?
5. Should I select the same options for feats/skills or could give companion another set of abilities?

If the latter is yes, then it become possible to use animal companion as skill monkey. Need Craft (Armorsmithing)? Easy! Release current companion, get new one with craft skills and PAO it to some form that could use craft.

[SangoProduction]

So, I should add extra HD for a companion

1. Would it change its size?
2. What feats could I select?
3. What skills considered cross-class and what are class skills for animals/elementals?

And what should I do if I released current companion from service and obtained new one?

4. Would it immediately get upgrade after coming to service?
5. Should I select the same options for feats/skills or could give companion another set of abilities?

If the latter is yes, then it become possible to use animal companion as skill monkey. Need Craft (Armorsmithing)? Easy! Release current companion, get new one with craft skills and PAO it to some form that could use craft.

1) depends on the animal.
2) Any that you qualify for.
Most DMs hand waive the issue. It finds you more than you find it. You could also find it at a zoo or other store, or have an adventure to yourself. That's all up to your GM.
3) Not a damned clue, since it's a type of creature not a class. Not even Savage Species has an answer. Ask your GM to waive class skills and that solves the entire problem.
4) as appropriate, yes.
5) It could (and probably should) have a new set of abilities.
If your DM is allowing obvious shenanigans, then sure. Thematically, animal companions are ones you have a bond with, not one you simply drop for a different set of skills. You probably would not be able to find it in a timely manner, and it'd be better to just buy the damned armor.

[eggynack]

3) Not a damned clue, since it's a type of creature not a class. Not even Savage Species has an answer. Ask your GM to waive class skills and that solves the entire problem.

The monster manual says, "All listed skills are class skills", so you can probably expect the companion to keep investing where they already have skills, and other plans seem like they'd cost you.

[Khedrac]

1. Would it change its size?
No - companion advancement Hit Dice explicitly do not count as normal advancement and do not affect the companion's size.

2. What feats could I select?
Those the DM approves - technically the companion is run by the DM not the player, however it is much easier for the DM to let the player run companions.

3. What skills considered cross-class and what are class skills for animals/elementals?
Those listed in the MM entry, whether or not they have ranks in those skills.

And what should I do if I released current companion from service and obtained new one?
4. Would it immediately get upgrade after coming to service?
Yes.

5. Should I select the same options for feats/skills or could give companion another set of abilities?
Entirely up to you - subject to DM approval (see above).