Birth of the Empires

So I'm happily playing, expanding nicely, then research a tech that allows me access to a better version of the colony ship.

Great you might think, however this new ship costs 4 times as much to purchase and uses a massive amount of titanium compared to the old one, with as far as I can tell the only useful upgrade being speed. OK not so bad, could be useful for far flung expansion.

The problem is that the old ship is gone from the build option, the ONLY option I have now for colonisation is a massively expensive ship.

Is this by design? If so then why? Why would being able to build a new ship make being able to build to the old design impossible? In effect what I have done is researched a tech that should make expansion easier only to find that it actually makes it massively more difficult.

Am I missing something here?

For all freighters, colony ships, scouts and for all ships of which there is a new version, the old one disappears from the build list. So I fill the queue with the old ones before the new one can be built.
In the case of colony ships, however, not only does the speed change, but also the terraforming speed and the number of buildings and colonists.

That's nice to know that its more than just speed, it doesn't change the fact that it's crippled my expansion and makes no sense.

If it makes sense is a question. I think it makes sense. But even if the ship is more expensive, the bigger terraforming speed is useful. And the better colony if you found a new colony is very useful. In fact as far as possible I try to found colonys with at least the second version of colony ships. (Some races like the Terrans have a third version too, but this version is often too late researched to be realy useful.)