If you have only one active power, can you still choose to replace it rather than placing a second power during your power up phase?
Pretty sure you can. During design we always said "Sure, why not?"
Unfortunately, that design philosophy got waaaaay out of hand towards the end:
Should we force the loser to get the winner a drink? “Sure, why not?”
K, thanks. A friend asked me this and I said something like "I think so, but I think it should almost never come up." The very next day I had proletariat sitting at full health (thanks to heals from Legacy) with 4 duplicates with only shared burden out, and thought it had become a liability rather than a benefit. Also useful for Baron Blade and anybody who makes a decision when a power comes into play (Unity, Dawn, Ra, Operative, etc.).
You definitely can if you so choose.