Can the mechanical servant from the UA class artificer be upgraded?

Question

Is it possible to upgrade the UA Artificer's mechanical servant (page 4 in the linked document)? The description of the servant doesn't mention this possibility in its description. For convenience purposes, the relevant text passage is presented here:

At 6th level, your research and mastery of your craft allow you to produce a mechanical servant. The servant is a construct that obeys your commands without hesitation and functions in combat to protect you. Though magic fuels its creation, the servant is not magical itself. You are assumed to have been working on the servant for quite some time, finally finishing it during a short or long rest after you reach 6th level.

Select a Large beast with a challenge rating of 2 or less. The servant uses that beast's game statistics, but it can look however you like, as long as its form is appropriate for its statistics. It has the following modifications:

• It is a construct instead of a beast.
  
• It can't be charmed.
  
• It is immune to poison damage and the poisoned condition.
  
• It gains darkvision with a range of 60 feet if it doesn't have it already.
  
• It understands the languages you can speak when you create it, butit can't speak.
  
• If you are the target of a melee attack and the servant is within 5 feet of the attacker, you can use your reaction to command the
  servant to respond, using its reaction to make a melee attack against the attacker.
  

The servant obeys your orders to the best of its ability. In combat, it rolls its own initiative and acts on its own.

If the servant is killed, it can be returned to life via normal means, such as with the revivify spell. In addition, over the course of a long rest, you can repair a slain servant if you have access to its body. It returns to life with 1 hit point at the end of the rest. If the servant is beyond recovery, you can build a new one with one week of work (eight hours each day) and 1,000 gp of raw materials.

As you can see it nowhere states that you can or can't upgrade it to improve the statistics you first picked. I personally think it is up to the GM's digression to allow it or not.

Answer 1

No, it cannot be upgraded.

Features in 5e only do what they say. The description doesn't say that you can → you cannot upgrade it.

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What you could maybe do at the DM's discretion, even though it's not explicitly stated, is changing its stats to that of another beast.
Note that this isn't possible as per RAW, but balancing-wise, I don't think it would be an issue. I would apply the same rules that apply when you build a new one - i.e. one week of work (8h/day) and 1000gp of raw materials.

That way, you can't, for example, just quickly change your servant from a tanky land-based creature to a stat-wise weaker creature with swimming speed on the go. But since you could have chosen the other form in the first place, spending 1 week and 1000gp on changing it shouldn't unbalance anything.

Answer 2

You technically can't change it but you have upgrades within the flexibility of roleplaying. It's a large beast under your complete control, no reason you can't pop a saddle on it and ride it into combat. AC too low; have you looked at barding the creature to give it some armor and upgrading its AC. You can technically have it made before the creature is even made - after all you're creating it and you just need to give the NPC making it the "Specs".

Where you would need to talk to your DM is if you start collecting items for modification. We're playing Curse of Strahd and I collected silver candlesticks. I could possibly smelt these down and use them to give my mechanical creature silver claws or fangs. Possibly...

The possibilities are limitless: think about what's allowed and if you start stretching the rules ask your DM.