I didn't know about this.

[ColinTH]

Playing as Montezuma on a ring map I crossed the land bridges and went straight to war. I researched iron working ASAP and upgraded my Jaguar warriors to swordsmen. After capturing some of the AI Cities I returned two of the swordsmen to combat lots of raging barbarians near my capital, there were some unexplored areas, and upon entering a ruins my swordsman was upgraded to a longswordsman. So it seems that if you have paid for one upgrade you can still gain another from a ruins!
I might possibly be the only forum member who was unaware of this fact, but just incase there is another plonker like myself out there I hope this is useful to you!

 

[eazyseeker]

Well, yeah, ruins can turn a manually upgraded unit into a more powerful version. What they can't do is upgrade a unit that was already upgraded throught ruins (to fix riflemain in 3000BC exploit/bug/thing)

 

[ColinTH]

What they can't do is upgrade a unit that was already upgraded throught ruins (to fix riflemain in 3000BC exploit/bug/thing)

That was truly diabolical and I used to play with a mod (the only time I have ever used one), that only allowed one upgrade per unit, thank God they fixed it.
On previous threads I have proposed that we shouldn't be able to get an upgrade from a ruins unless we have researched the technology, needless to say my proposal was not met with very much aproval!