Colossus isn't really replaced by SETI.
SETI only boosts science, where Colossus adds trade arrows to the city. Those trade arrows are then used when computing the tax, luxuries, and science the city generates.
Trade arrows are also used when computing the delivery payment for our freight, as well as the per turn bonus that is given to the city for each active trade route.
Leo's will not be upgrading our troops, but the ones of those that we get when buying other Civs cities. Is that benefit outweighted by the extra trade, tax, and science we will get from colossus?
Normally, what I do, is disband any ancient troops from a newly purchased city in order to use them to get shields into the production box. Then, I would PRB a needed city improvement. If so desired, they could be used to IPRB a new military unit in the city. Since units are only upgraded one 'level' each time a tech is discovered, it can take 2 techs before an archer is made into a rifleman. That could be about 6 turns. So, we would have to either keep the archer pushing forward with our 'normal' troops waiting for it to upgrade and be useful, or leave it behind, wait for it to upgrade, then try and have it catch up to the nomal troops.
I think the economic benefits of the colossus outweighs the benefits of outdated units being upgraded. Plus, Automobile will give us superhighways that will increase our delivery payouts even more. In addition to that, we can get to even better ground units after getting automobile.
The flip side to that is that air units can attack without fear of being counterattacked since the AI does not have any air techs (and hopefully will never get them).