Learn English – How to understand this film name ‘Tron: Legacy’

Develop has many different meanings in English. Mostly it could mean to bring to a more advanced state, cause to grow or expand, to elaborate or expand in detail, or to bring into being. There are many more specific meanings depending on domain.

Upgrade has a relatively more narrow definition, and means to improve something that's old or outdated, or to raise in value or esteem.

You could possibly find one where upgrade could be used instead develop, but it would be difficult, and it would be in only one specific meaning. Develop has the sense of an ongoing process, upgrade is discrete. You upgrade something from one stage to another. You develop something from a less advanced to a more advanced state in a gradual process.

To use your example, an economic upgrade would be a something (very specific) that happened to the economy that could be precisely measured as better. An economic development would be something added to the economy that is new, or depending on context, you could be speaking of the economy made generally better through several changes.

Another way of thinking of the difference, when you "upgrade" something you are literally or figuratively replacing something with something improved. When you "develop" something, you are changing something (usually improving it), or creating something new from scratch (not replacing anything.)

It's interesting when I was searching for usage, one example I found of the phrase "economic upgrade" happens to be an obvious translation, so "economic development" would have been better…

While the first translation, stating that the time for love is very short, so it's such a pity. The other translation, looks more blur, as it's something like: we only have time to love, and everything is fleeting.

The passage quoted in your question was:

There isn't time -- so brief is life -- for bickerings, apologies, heartburnings, callings to account. there is only time for loving -- & but an instant, so to speak, for that.

Deconstructing that passage:

There isn't time -- so brief is life -- for bickerings, apologies, heartburnings, callings to account.

The narrator states that life is brief, and because life is brief there isn't enough time "for bickerings, apologies, heartburnings, callings to account."

The phrase would have made more literal sense if it read isn't enough time but it's a common idiom to drop enough. There is time, there just isn't enough time because life is brief and therefore what little time there is, is precious.

Too precious to waste on "bickerings, apologies, heartburnings, callings to account", things that do not enrich one's life.

There is only time for loving --

So what is there time for? There is time for loving, implicit in this is the idea that the narrator finds love to be one of the most worthiest of human pursuits (because he has already stated that life is all too brief, which implies time is very precious).

& but an instant, so to speak, for that.

This is a magnification of the idea that time is brief (repeated here for poetic effect and emphasis) - even though there is time for love, that time is still but an instant. This serves to stress just how short a human life is, and also stress how worthy a pursuit the narrator finds loving to be.

There is an implicit admonishment here for the reader to seize the moment when loving, because in the blink of an eye there will not even be enough time for that.

The second translation you mention:

We only have time to love, and everything is fleeting.

Is less poetic and carries less power, but the essential idea is the same.

We only have time to love

Implies we should do nothing but love, which carries the same idea that love is the noblest of human pursuits that we gleaned from the first translation. We would never read this phrase to literally mean there is not enough time to do anything else but love, but instead we read it to mean the time we have is too valuable to spend doing anything else but love.

Everything is fleeting

Means everything passes quickly, this phrase being so close to the phrase life is brief encourages us to think about the shortness of a human life, but also we are reminded about the poignant fact that even once the prize of love is attained it will only last a short while.

But compare with this first passage:

time [is] but an instant

This is a more powerful rendering of the idea, time is not only fleeting, but is a mere instant. We should seize it and (in relation to the rest of the passage) spend what little time we have in the pursuit of love.