There was recently a LOT of interest in this subject.
As a City Builder, I was sad to see beginners wasting their Hard Earned Gems even considering doing Building Upgrades
MY OPINION is that if you are not COLLECTING Special Buildings, STAY AWAY from this Feature! If you are COLLECTING, GREAT, UPGRADE, Put it aside and keep it FOREVER!
If you are BUILDING your CITY, You are WASTING your HardEarned Gems EVERY Time you UPGRADE AND there is NO MATH you can do to change that FACT!
I will give you an example-
I had a T9 DiamondHall (Cost500Gems) I got an Offer UPGRADE to T10 (Cost1000Gems) ! So, If I accepted the Offer, I would get a T10- Angel Tower, I think? ALL T10s are Worth 1000Gems in Your City! AND I LOSE My 500Gem T9 DiamondHall! GREAT for COLLECTORS, BAD for BUILDERS!
For those 1000Gems, I could Keep my T9 DiamondHall(500Gems), Buy Another DiamondHall(500Gems), (2) T8 Buildings(2X200Gems=400Gems) and (2) T6 Buildings(2X50Gems=100Gems)! I get a 4DayHMP on those! When I MonMake the (2)T6 to L1, I get another 2DayHMP on it, When I Evolve the L1(100Gems) to L2 with my (1)T9 and (2)T8s, I get 2DayHMP on it! ALL The time SuperPowering MY TB, LOL! And I still got my Original T9 DiamondHall!
We are all FREE PPL here. We can enjoy ourselves as We see fit. If you are COLLECTING Special Bldgs, More Power to ya! If you are BUILDING your CITY? Beware of UPGRADING!
Peace and Love-Hugs and Handshakes Yall!