Tips for replacing obsolete units?

[Egregius]

Well as general strategy I usually make sure I have a lot of planets defendable with a few key points, and I put all mines/fighters/sattelites at those keypoints. I also build some WPFs and troops on the other planets, and keep obsolete fighters/sats as extra backup def on them.

Usually all that's good for is population happyness (troops, any troop will do) and ease of mind (well even obsolete wpfs can do some damage).

I only upgrade planets 'at the front' or in systems with possible enemies, and the rest I upgrade only once or twice a game.

Why scrap obsolete sats/fighters anyway except when you forgot to put the units limit up?