Not sure how to explain in words, so I'll give you a shitty mspaint image:

When you mouse over an improvement - either in the list of those available to build or those already built - a tooltip pops us to show all versions that it can be upgraded into, and always including the first one that needs to be built. The level of improvement upgrade that is available to you with your current techs is highlighted in the tree, and all details pertinent to each level is displayed against each one.

Going even further, a list of technologies required to unlock each level could also be shown, with the ones that you have and do not have identified by way of a colour coded list.

It looks good in my head at least, and I think it would be very helpful - as any addition that presents necessary information to the player always is - especially to new players who are not yet familiar with the entire tech tree and potential unlocks that they could get.