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Here's an article titled "NBN rolling out FttDP to 700,000 premises, replacing Optus HFC footprint":

The National Broadband Network (NBN) company has announced that it
will be replacing the Optus hybrid fibre-coaxial (HFC) footprint with
its fibre-to-the-distribution-point (FttDP) network, with up to
700,000 premises to be covered by the new network technology.

...

DOCSIS 3.1 -- currently due to be launched in the second half of
2017 -- would enable HFC users to reach speeds of 1Gbps down/100Mbps
up.

...

In terms of FttDP speeds, the VDSL box currently being used has the
capability to deliver speeds of up to 500Mbps, but, once it is updated
to a G.Fast box, has the capacity for 1Gbps.

In the last sentence, there are two main clauses connected with but, and the verb has of the second main clause is in the simple present tense and denotes a scheduled future event.

Is this use of has grammatical?

What if has is replaced with gets?

In terms of FttDP speeds, the VDSL box currently being used has the capability to deliver speeds of up to 500Mbps, but, once it is updated to a G.Fast box, gets the capacity for 1Gbps.

In another question "Very very confused! which verbs can be used in simple present tense for scheduled future events?", the only answer there says, "Almost any action verb can be used in the present tense for a scheduled future event."

Does this mean that stative verbs such as has in the above article cannot generally be used in the present tense for a scheduled future event when used in a main clause?

"will have" is clearer/better/more correct. "gets" is terrible. As is, the meaning is understandable, but not grammatically ideal according to my Western Canadian dialect. It would be fine to say that a G.Fast box has the capacity for 1Gbps (or perhaps "has a capacity of 1Gbps" would be better), but saying once X happens, Y applies, is not proper English IMHO, unless you are talking about general statements, like "When it rains, I (generally) use an umbrella". The sentence in question is not meant to be interpreted as a generic statement, hence the clash in grammar. In fact, I had to use "When it rains, in my example." Once doesn't seem to be very amenable to generic statements.

The examples in the other question you referenced are not generic statements, but they are also not qualified by a conditional statement, like your example is. You can say "I speak tomorrow at 3pm," but you can't say "If my paper is accepted I speak tomorrow at 3pm." At least, that sounds odd. The second clause is in the subjunctive mood, I believe, so it wants a modal (will). Simple present is not right.

Upon further consideration, it sounds like whoever wrote that might be trying to say that the VDSL box has (presently) a latent capacity for 1Gbps, but it cannot be realized until another component is upgraded? If that is the case then maybe there is some warrant for the present tense, but then it could be said more clearly.

You seem to be distraught that the VDSL box, a currently used piece of hardware, once updated to a G.Fast Box, has a capacity of 1Gbps.

From the customer's point of view, this update and its resultant speed increase is a future event and could be announced:

After an update of the currently used VDSL box into a G.Fast Box, customers will enjoy a capacity of 1Gbps.

From the company's point of view, however, this is simply an established fact in the present tense. They tried it out in a lab somewhere. We've got this box, we do a firmware update and voilà, we've got 1Gbps capacity.

By the way, gets the capacity in your suggested revision sounds non-idiomatic: will get does not, because, as you noted, have is statal, but get can be a process.