Summoning animal companions:
Summoning a new animal companion requires:
'8 hours of work and the expenditure of 50 gp worth of rare herbs and fine food'
whereas, re-summoning a slain animal companion requires:
'8 hours of work and the expenditure of 25 gp worth of rare herbs and fine
So re-summoning your slain companion is 50% cheaper than summoning a new one.
Managing ASIs for animal companions:
Once summoned, the Animal Companion rules state:
your animal companion gains all the benefits of your Companion’s Bond ability.
And, Companion’s Bond states:
Your animal companion has abilities and game statistics determined in part by your level.
One outworking of which is ASIs:
Whenever you gain the Ability Score Improvement class feature, your companion’s abilities also improve. Your companion can increase one ability score of your choice by 2, or it can increase two ability scores of your choice by 1. As normal, your companion can’t increase an ability score above 20 using this feature unless its description specifies otherwise.
So, what does this all mean for your questions?
Every time you summon a new companion you're effectively creating their character sheet from scratch, choosing a template animal to start with, then applying all of the appropriate features gained via Companion’s Bond, some of which are 'determined in part by your level' - which includes ASIs.
Just as with PCs, there's no rules for changing an animal companion's ASIs later, after they've been assigned. Once they're assigned to a particular companion they're locked in and can't be re-allocated to that same companion later. So, if you re-summon a slain companion, it's stats will be the same as they were previously (with any extra improvements added, if you've levelled up since their death).
However, if you summon an entirely new animal companion there's no rules stating that you need to spend the ASI's in the same ways as you did previously. That's true whether you switch to an entirely different animal, or just want to summon a different wolf after you realise that putting all your old wolf's ASIs into CHA hasn't helped it's combat utility much.
The extra flexibility you get when you summon a new animal companion, instead of re-summoning an old one, is accounted for by the extra material cost you spend.
Or putting it another way: imagine if your Barbarian PC died and your DM told you that they were fine with you rolling up a Wizard instead, as long as you still put all of your ASIs into STR and CON? That would be a pretty damaging hindrance to your new character. The game doesn't expect you to treat PCs that way and it doesn't expect you to treat animal companions that way either.
All references are from Unearthed Arcana: The Ranger, Revised pp. 5-6.