5 ➡️ 6 only requires building blocks.
20 ➡️ 21 requires having 10 level 5 Presidios and a top 800 rank, but no downgrades.
IIRC, upgrading from milestone levels to the next ones does not require downgrades of others' Bastions (makes sense, since you always need to downgrade a Bastion of the same or higher level).
Downgrade swap works like this:
- You are both at level n.
- You downgrade theirs to level n - 1.
- They rebuild to level n and you upgrade to n + 1. Do not mess up this step!!!
- They downgrade you to level n.
- You rebuild to n + 1, while they upgrade to n + 1.
The order is important!!! Upgrading to level n + 1 removes the requirements up to that level. But, if you swap steps 3 and 4 above and get downgraded first (from n to n - 1), then the fulfillment of the downgrade requirement doesn't stick and the first downgrade is instead wasted.
Notice that this becomes problematic with milestone levels. Say you are both at level n = 19. Then the above looks like this:
- You are both at level 19.
- You downgrade theirs to level 18.
- They rebuild to level n and you upgrade to 20.
- They downgrade you to level 19... Whoops! This won't work, because 20 is a milestone level that cannot be downgraded.
So, instead, you need to upgrade to n + 2 (in this example, 21), so they can downgrade you to n + 1 (in this example, 20). This, however, usually means holding 10 more Presidios.
I strongly advise you to plan ahead in a way that will allow you to boost each and every thing that you need to do (including upgrading temporarily held Presidios, if you need them) with rubies. Time is of the essence, unless you want to risk someone else crashing your party.
This post is courtesy of my mates in Phoenix@s2. I was unsure of the details, so I asked them. 🤓
Good luck!