Upgrading units

[robfield]

Cannot upgrade bomber to stealth bomber in city where i can build stealth bomber

[Skanky Burns]

That would be because bombers can not be upgraded to stealth bombers, ever. Just build new stealth bombers, and either disband your existing bombers to help speed up production of the new bombers, or just keep using obsolete bombers.

If they keep being shot down, you will eventually only have stealth bombers anyway, and if they arent being shot down, theres no real reason for making an stealth-bomber-only airforce.