Another Trello dev here.

Basically, we have many clients that are consuming the API, including our mobile apps. We never change the published interface so that if you're getting some board fields that will always work. This means that we don't change the types of fields (for example string vs object) or the names of fields since that changes the published service contract.

What we do instead is add more fields. A very good example of this is emoji. We added emoji support a while back and instead of changing the "text" fields to be objects or embedding HTML (gah) into them we added another field called "textData" that has the extra info.

This is copy/pasted from an actual call for getting card actions:

        "textData": {
          "emoji": {
            "daft": "https://trello-emoji.s3.amazonaws.com/4f820a995a03e8e82d134ac4/ae846ca70bf7aa95dd3330b8fd1c70cb/art-daft-punk-steampunk-951631.gif"
          }
        },
        "text": "Custom emoji! :daft:"

This may not be sustainable in the long term and if it's not we'll version the API if we have to at that point. So far though, the API has proven itself to be very well designed and adaptable (thanks to @d_lec)

+100