New Railroad Gun

Level 2 is more expensive but stronger and unlocks later.

Hey, I've researched level 2 railroad gun in a game.. but my existing level 1 railroad guns I already built.. It's not even giving me an option to upgrade them to level 2 like other units?? I can build new level 2 railroad guns..but how do I upgrade my existing level 1 railroad guns? Strange...never ran into a situation where it doesn't even given an option to upgrade an existing unit to a higher level that I've researched