Classical Chinese grammar help: 蓋將自其變者而觀之，則天地曾不能以一瞬 The second clause is translated as some version of: "If you look at it from the point of Change, then heaven and earth never stay the same for even a moment".

Here's what I think I know:

不 negates a verb
能 indicates that something is possible, and functions as an auxiliary to another verb
不能(verb) therefore means "cannot (verb)", "does not (verb)"
以 has a variety of meanings, but the only one that fits here is to indicate a period of time: 一瞬 ("a moment", "the blink of an eye").

My question is this: what verb is 不能 modifying? What in the verse conveys "to stay the same"? Is there an implied verb somewhere meaning "to be unchanged"? How is the above interpretation derived, exactly?

Many thanks to anyone who can explain this to me.

I should mention also that I first encountered this in its literary Japanese form, where the full gloss is: 蓋し将た其の変ずる者よりして之を観れば、則ち天地も曾て以て一瞬なる能はず, if that helps anyone to understand the issue. There is no verb explicitly indicating "existence", "stopping", or "lack of change" in the Japanese interpretation either.
First, I'm very surprised that a foreigner is even reading the classical article 前赤壁賦.

For your question, I think the answer is related to omission,  a common grammatical phenomenon in classical Chinese.

Here, 以 doesn't mean "to stop". When it means to stop, 以 is just an alias of 已 (another common phenomenon, called 通假, where characters with similar pronunciations or shapes are used  interchangeably) , which means to stop. If 以 really means to stop here, what  is the role of 一瞬? 

To stop a moment doesn't make sense.  From the context, the author 苏轼 wants to express that "the universe is always changing. Changes never stop".  If it means to stop within a moment,  the meaning sounds weird: "the universe CAN'T STOP within a moment". So does  it takes some time for the universe to stop? 

I think the sentence omits the complement: 则天地曾不能以一瞬 观之/度 (duo 2nd tone, to consider, to think of)之. Then the universe can't be even  observed / considered in (the unit of) one moment, since it changes so fast. Even in one moment (very short period), it has changed a lot, and you can notice no details or nothing meaningful.

Why is there such an omission? Well, the pattern of this article is 赋, and it requires the writer to use a lot of 对仗 ( coupling sentences. Two groups of sentences written in the similar pattern. For example, same numbers of characters, similar sentence structure).  You can notice a lot of coupling sentences in this article, can't you? The next sentence is coupled with the sentence in the questions, so the number of characters is restricted here. Also, the verb in the complement repeats 观之 in the previous clause.

PS: I'm not a linguist, so please correct me if I use incorrect terms in the answer. I'll appreciate that.