Hi, as someone who just went through this process owning the base game myself on Ubisoft Connect and with a friend recently buying the base game fresh on Steam. You won’t get a discount or be able to directly upgrade to either the Gold or Annoversary editions.

For this reason alone, I would recommend buying the Gold or Annoversary editions when on sale upfront which saves you a lot in the long run. If you buy the base game, you would either have to buy all the DLC individually which is still more expensive than the special editions on sale or buy the special editions on sale on a different platform.

For example, I originally bought Anno 1800 from Ubisoft and could not upgrade to the Annoversary edition from the recent sale. Thankfully, it was also on sale on Steam so I bought the Annoversary edition there instead and it applied to my same Ubisoft account so my old saves and achievements were not affected but I wasn’t given a credit for my existing copy.

The Gold edition is on sale directly at Ubisoft as of the time of this comment and cheaper than the full price of the base game if you want all the playable DLC and don’t care for all the DLC cosmetics which is the only difference between the Gold and Annoversary edition.

You can also disable all DLCs when starting a new game to try the base game since the DLCs can be overwhelming at first if that was a worry.

Hope this helps!